MAR-155: Make the ansible_key parameter in the bootstrap role mandatory:

- Updated the preseed role.
- Removed default value for the parameter.
- Updated tests.
- Updated role reference documentation.
- Updated release notes.

MAR-129: Updated bootstrap role for Ansible 2.5.x and new Molecule:

- Include top-level lint configuration file.
- Fixed linting errors.
- Moved test playbook parameters into group_vars.
- Fixed Ansible deprecation warnings.
- Updated how the hosts are referenced in tests.