added unittest for compiler/lexemes.py

[Boot-Error]

Added unit tests for functions defined in compiler/lexemes.py based on #25

For function parse_for_variable_hierarchies, the test considers 2 cases.

1. The data is a CommentedSeq with a CommentedMap with __from__ key in it.
2. The data is a CommentedMap with __from__ key in it.
3. The data is a CommentedMap with a CommentedMap with __from__ key in it.

[maany]

Good work @Boot-Error . Let's try to one-up this test case ;)
Essentially, we can read from a sample site_level_config_file.yaml which contains different use case scenarios for the from keyword.

global_variables:
  - &spark_master_ip_address 188.184.104.25
  - &spark_master_fqdn simple-lc01.cern.ch
  - &spark_worker_ip_address 188.184.30.19
  - &spark_worker_fqdn simple-lc02.cern.ch
  - &spark_submit_ip_address 188.185.84.189
  - &spark_submit_fqdn simple-lc03.cern.ch
  - &spark_level_2_configurator sh
  - &l2_configurator:
    __from__: *spark_level_2_configurator

spark_master_runtime_variables:
  - &master_runtime_var_host
    __from__: *spark_master_fqdn

spark_worker_runtime_variables:
  - &spark_worker_runtime_var_host
    __from__: *spark_worker_fqdn
  - &spark_worker_runtime_var_spark_master
    __from__: *spark_master_fqdn

spark_submit_runtime_variables:
  - &submit_runtime_var_host
    __from__: *spark_submit_fqdn

preferred_tech_stack:
  level_1_configuration: puppet
  level_2_configuration: 
    __from__: *l2_configurator
  container_orchestration: docker-swarm
  container: docker

We can use PyYAML to parse this file into a python dict and pass it to the parse_variable_hierarchies function.
We can test the output the the parse_variable_hierarchies function to check if the variables that were assigned values using from get the correct values. Let me know if anything needs clarification.

You can incorporate this once you have drafted your proposal ;) Good Luck.

[Boot-Error]

If the config is loaded as a dict, then parse_variable_hierarchies function returns the same dictionary (first if condition).

So it is also important to load the same file using ruamel's yaml loader to test if the CommentSeq and CommentMap conditions are functioning too.

And I'm having trouble with loading the same config using ruamel's loader. It fails at __from__: *l2_configurator. Here is the full stack trace:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/booterror/Development/Contributing/gsoc19/simple_grid_yaml_compiler/venv/lib/python2.7/site-packages/ruamel/yaml/main.py", line 325, in load
    return constructor.get_single_data()
  File "/home/booterror/Development/Contributing/gsoc19/simple_grid_yaml_compiler/venv/lib/python2.7/site-packages/ruamel/yaml/constructor.py", line 106, in get_single_data
    node = self.composer.get_single_node()
  File "/home/booterror/Development/Contributing/gsoc19/simple_grid_yaml_compiler/venv/lib/python2.7/site-packages/ruamel/yaml/composer.py", line 78, in get_single_node
    document = self.compose_document()
  File "/home/booterror/Development/Contributing/gsoc19/simple_grid_yaml_compiler/venv/lib/python2.7/site-packages/ruamel/yaml/composer.py", line 101, in compose_document
    node = self.compose_node(None, None)
  File "/home/booterror/Development/Contributing/gsoc19/simple_grid_yaml_compiler/venv/lib/python2.7/site-packages/ruamel/yaml/composer.py", line 138, in compose_node
    node = self.compose_mapping_node(anchor)
  File "/home/booterror/Development/Contributing/gsoc19/simple_grid_yaml_compiler/venv/lib/python2.7/site-packages/ruamel/yaml/composer.py", line 217, in compose_mapping_node
    item_value = self.compose_node(node, item_key)
  File "/home/booterror/Development/Contributing/gsoc19/simple_grid_yaml_compiler/venv/lib/python2.7/site-packages/ruamel/yaml/composer.py", line 138, in compose_node
    node = self.compose_mapping_node(anchor)
  File "/home/booterror/Development/Contributing/gsoc19/simple_grid_yaml_compiler/venv/lib/python2.7/site-packages/ruamel/yaml/composer.py", line 217, in compose_mapping_node
    item_value = self.compose_node(node, item_key)
  File "/home/booterror/Development/Contributing/gsoc19/simple_grid_yaml_compiler/venv/lib/python2.7/site-packages/ruamel/yaml/composer.py", line 138, in compose_node
    node = self.compose_mapping_node(anchor)
  File "/home/booterror/Development/Contributing/gsoc19/simple_grid_yaml_compiler/venv/lib/python2.7/site-packages/ruamel/yaml/composer.py", line 217, in compose_mapping_node
    item_value = self.compose_node(node, item_key)
  File "/home/booterror/Development/Contributing/gsoc19/simple_grid_yaml_compiler/venv/lib/python2.7/site-packages/ruamel/yaml/composer.py", line 116, in compose_node
    None, None, 'found undefined alias %r' % utf8(alias), event.start_mark
ruamel.yaml.composer.ComposerError: found undefined alias 'l2_configurator'
  in "<byte string>", line 29, column 15:
        __from__: *l2_configurator
                  ^ (line: 29)