MAR-150: Use Gimmecert for X.509 test fixtures in the common role:

- Drop the statically-generated X.509 certificates used for testing.
- Introduce fixture playbook in preparation phase that sets-up the
necessary certificates.
- Update the tests to use the fixture.
- Dynamically calculate the OpenSSL hash for CA certificate instead of
having the hard-coded value.
- Add the fixture artefacts to .gitignore files.

MAR-148: Drop configuration of retry files location:

- Ansible 2.8.x has switched to not creating the retry files by
default.

MAR-129: Minor preparation for updating Ansible version:

- Ignore the pytest_cache directory in Git.
- Updated Ansible to latest 2.3.x version in requirements file.
- Make sure the SSH client private keys have correct permissions when
testing the backup_server role (must be 0600), otherwise we get
errors from SSH due to insecure permissions during testing.
- Updated test for testing installed packages in the wsgi_website
Python virtual environment to test against up-to-date
packages. Might be nice to fix this one with something less hacky in
the long run.

MAR-128: Starting to move towards Molecule 2.x:

- Updated requirements and its input file, primarily for use of
Molecule 2.4.x.
- Updated gitignore to exclude some additional artefacts produced by
Molecule 2.x.

MAR-110: Implemented script for running tests for all roles:

- Updated gitignore file to ignore test reports.
- Updated instructions for running tests.
- Added Bash script that wraps around Molecule and allows output of reports into
a directory.

MAR-30: Implemented tests for role 'preseed':

- Added global flake8 configuration to allow up to 160 characters per line (for
Python linting checks).
- Updated gitignore file to ignore Molecule artefacts.
- Role is tested against 64-bit Debian Jessie and 64-bit Debian Stretch.
- Tests cover variations on optional parameter inclusion, as well as generated
preseed files.