MAR-189: Refactored web_server tests to be less hostname-dependent:

- Use the inventory name (alongisde distribution version) in places
where it's possible and makes sense to do so.
- Rearrange the IP allocation a bit to make more sense.
- Drop architecture information from the hostnames.
- Make the connectivity test operate over a matrix of input
parameters.
- Replace singular use of wget for testing with curl.

MAR-163: Ensure host.run is not suspectible to shell injection:

- Use host.run's built-in capability for handling parameter escaping
and insertion.

MAR-132: Added support for Debian 9 (Stretch) to web_server role:

- Introduced internal parameters for controlling differing package
names, service names, and paths for PHP FPM package.
- Added Debian 9 machines to Molecule configuration, including the
client machine.
- Restructured slightly preparaiton playbook to support both Jessie
and Stretch.
- Added custom pytest fixture for having a better way to determine
expected package names etc related to PHP.
- Created copy of private key/certificate pair used for testing of
mandatory parameters (to be used with Stretch machine).
- Fixed invalid specification for hosts on top of which the
connectivity test should be run.
- Updated a couple of task names (avoiding to reference PHP 5).
- Updated documentation.

MAR-132: Added support to xmpp_server role for Debian 9 (Stretch):

- Updated tests to include Debian 9 in testing. Existing private keys
are reused where possible (since most of the naming is identical
between the machines with jessie/stretch).
- Updated invocation of sendxmpp in tests as workaround for
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=854210.
- Updated testing of imported keys to accomodate differences between
gpg/gpg2 (used by apt-key in Jessie/Stretch).

MAR-129: Updated web_server role for Ansible 2.5.x and new Molecule:

- Fixed linting errors.
- Include global linting configuration file in the test configuration.
- Moved variables from the test playbook into group variables.
- Updated how hosts are referenced in tests.
- Fixed deprecation warnings.

MAR-128: Upgraded tests for ROLE_NAME 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).
- Switched to extracting hostname instead of hard-coding it in a
couple of tests.
- Renamed hosts instances to include Debian version in their
hostnames.
- Fixed some linting issues.