Support variables for different models within groups

[mjbnz]

Pre-Request Checklist

• Passes rubocop code analysis (try rubocop --auto-correct)
• Tests added or adapted (try rake test)
• Changes are reflected in the documentation
• User-visible changes appended to CHANGELOG.md

Description

This pull request allows for the specifying of vars for models within groups, overriding more general vars:

groups:
  foo:
    models:
      arista:
        vars:
          ssh_keys: "~/.ssh/id_rsa_foo_arista"
      vyatta:
        vars:
          ssh_keys: "~/.ssh/id_rsa_foo_vyatta"
  bar:
    models:
      routeros:
        vars:
          ssh_keys: "~/.ssh/id_rsa_bar_routeros"
      vyatta:
        vars:
          ssh_keys: "~/.ssh/id_rsa_bar_vyatta"

[mjbnz]

This branch is on top of my 'rubocop-fixes' branch, hence the extra commits.

[mortzu]

Can you reflect the change in the documentation please?

[mjbnz]

Can you reflect the change in the documentation please?

Done - however this is a less than ideal way to document this particular change, but doing it correctly requires the documentation rewrite (it should really be in a yet-to-be-created "Variables" section of the docs).

[mortzu]

Please fix the merge conflict

[mortzu]

Please fix the merge conflict (again...sorry)

[TafkaMax]

Does this allow overriding username and password from all other sources?

My question is specified here aswell: #3027