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-194: Switch to using nmap for testing connectivity in common role:

- Unfortunately, hping3 does not support IPv6.

MAR-191: Make tests for allowed/disallowed connections more generic:

- Use dynamic inventory to retrive the list of hosts instead of
hard-coding release names in the tests.

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-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.