Fixes #37327 - GUI/API to allow importing/overriding of Ansible variables directly from YAML/JSON

[Thorben-D]

This PR adds a new wizard to Configure > Ansible > Variables to allow users to import/override Ansible variables from YAML-files.

Key features:

• This wizard may also be opened from Configure > Ansible > Roles to directly select a role
• Variable name, type and default value may be changed in the UI
• Users may select specific variables to import from a file
• Should a variable duplicate an already created variable, the user will be warned and if he chooses to proceed, the variable is correctly marked as overridden
• Methods to import variables from base64-encoded YAML files or JSON are exposed as API endpoints

Note: Currently, from_yaml and from_json are not properly documented. They require a dictionary as a parameter and I am unsure how to handle this with apipie (if it is even supported).

Documentation Pull request: theforeman/foreman-documentation#2941
Depends on: #696