Allowing mixing links and inventory variables

[andycui66]

We can use links to specify some default values for group_vars and host_vars. This PR gives user ability to override default values within molecule.yml file. This align with Ansible variable precedence convention.

Inspired by #3501 and fixes #2759

A few people need this feature including myself but original ticket was closed because issue opener found another solution. I am trying to pickup the work.

[gardar]

What's the status of this?

Will this result in the following variable precedence? (Ordered from least to greatest)

1. provisioner['inventory']['links']['group_vars']['all']
2. provisioner['inventory']['links']['group_vars']['groupname']
3. provisioner['inventory']['group_vars']['all']
4. provisioner['inventory']['group_vars']['groupname']
5. provisioner['inventory']['links']['host_vars']['ansible_host']
6. provisioner['inventory']['host_vars']['ansible_host']
7. ...followed by normal ansible role/playbook precedence