- create a new NamedTuple class to be the COU Namespace. With that,
  type is still checked and few arguments are necessary to pass to
  the functions that use the arguments passed by the user. Moreover,
  docstrings are shorter and easier to maintain

- Global variables for "data-plane" and "control-plane".

- check if control-plane is already upgraded if user pass "data-pane"
  upgrade group
- check if data-plane apps exists if user pass "data-pane"
  upgrade group
- splitted the logic on determine_upgrade_target to several functions

- fixed run_upgrade unit test

- added expected error messages on the remaining sanaity checks
  unit tests.

- small change in is_highest_release_achieved function

  a boolean.
- removed the pre_plan_sanity_checks is_valid_openstack_cloud to
  be nested under determine_upgrade_target
- fixed unit tests to test the verify_* functions instead of the
  pre_plan_sanity_checks