MAR-232: Switch to using IP addreses in firewall rules for mail_forwarder role:

- Perform lookups on managed machine for the passed-in SMTP relay
host, and use those values when populating the firewall rules.

MAR-192: Fix missing dnspython library as requiremnts for running the roles:

- Turns out there is still one usage left in the mail_forwarder role.
- It might be a good idea to try to get rid of that single usage to
make the firewall coming up more reliable during boot time.

MAR-192: Get rid of the dnspython requirement, and document netaddr requirement:

- This is the result of previous deprecation of using resolvable names
for setting-up the list of allowed incoming connections when in
maintenance mode.

MAR-152: Update documentation regarding removal of support for Debian 8 Jessie:

- Updated introductory information about the project.
- Removed references to Debian 8 Jessie from documentation.
- Updated release notes.

MAR-125: Switching to Ansible 2.3.x:

- Updated documentation to reference only Ansible 2.3.x, removing mentions of
Ansible 1.9.x, and removing comments related to delayed switch to Ansible 2.x.
- Updated usage instructions to use Ansible 2.3.x.
- Updated pip requirements and its input file to use up-to-date versions of
Ansible and Molecule.
- Updated test site Ansible configuration to use pipelining.
- Fixed missing brackets in test site tls.yml playbook for "with_items.

MAR-30: Added documentation related to development:

- Moved existing test site documentation under the new 'Development' chapter.
- Added instructions for preparing development environment.
- Added instructions for running Ansible role tests.
- Added requirement input and txt files for use with pip-tools.