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-149: Switch to using File.content_string instead of File.content in tests for all roles:

- The .content variant returns contents as bytes, while the
content_string returns a string (which is what is wanted in
practically all cases).

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.

MAR-128: Upgraded tests for bootstrap role:

- Switch to new Molecule configuration.
- Updated set-up playbook to use become: yes.
- Moved some preparatory steps outside of the main playbook (eases
idempotence tests).
- Updated tests to reference the yml inventory file.
- Updated tests to use new fixture (host instead of individual ones).
- Fixed some linting issues.