MAR-241: Implemented connection rate limit tests for the common role:

- Tweak some test machine parameters to have better test
sample representation.
- Tests heavily rely not being laggy to get the expected output. Tests
may need to get reworked a bit if they end up failing repeatedly
during test runs.

MAR-192: Switch to using NTP pools instead of servers:

- This is the recommended configuration by NTPsec, and also default on
Debian. Previuosly suggested values for servers have been pool
addresses in any case.

MAR-194: Use IP addresses instead names for maintenance allowed connections:

- Less ambigious. Solves problems around names being resolvable to
different IPs depending on what DNS server is used.
- Parameter renamed to better represent what is specified.
- Updated requirements to allow execution of ipv4/ipv6 filters.
- Pin the rich requirement to a lower version for compatibility
reasons.
- Implement tests for IPv6 connectivity tests.
- Improve rendering of base rules (indentation).

MAR-154: Drop support for Python 2.7 from the common role:

- This primarily concerns removal of pip requirements upgrade checks
for Python 2.7.
- The old Python 3-specific paths are getting deprecated, and the
Python 2 paths will be used instead.
- Set permissions on pipreqcheck directory explicitly (because of
deprecation testing).

MAR-189: Added support for Debian 11 Bullseye to common role:

- Drop the architecture suffix from all machines.
- Introduced a hack/patch for Ferm utility that disables use of legacy
iptables tools.
- Minor fixes around package name differences compared to older
distribution.
- Use versioned binary name for Python 2.7.. Bullseye does not
default/have /usr/bin/python pointing to Python 2.7 binary.

MAR-182: Added ability to specify input requirements for running pip package upgrade checks against helper virtual environments:

- These environments are used for running the package upgrade checks
themselves.

MAR-181: Dropp support for Debian 9 Stretch from the common role:

- Drop platform name from all helper VMs.
- Switch to using IPs from VirtualBox default allowed host-only
network subnets.

MAR-151: Added support for Debian 10 Buster to common role:

- Updated tests.
- Updated role reference documentation.
- Updated role metadata information.
- Refactored IP plan for the test machines for better separation
between different types of machines and versions.
- Parametrised tests for limited connectivity using the maintenance
mode.
- Don't use MariaDB compat package in tests - name differs between
Debian 9 and Debian 10, and relevant parameter is already getting
tested properly using the remaining packages.

MAR-160: Added maintenance and maintenance_allowed_hosts parameters to common role:

- Lets the user specify list of hosts for which the incoming
connections should be allowed.
- Defaults are not to limit connectivity.
- Implemented the necessary tests.
- Set-up the base ferm/firewall rules if maintenance mode is enabled.

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-129: Updated backup_server role linting and test configuration:

- Fixed linting issues.
- Use global linting configuration file.
- Moved test variables into group_vars.