Update unit testing strategy

[tcuthbert]

Outline

An update of the unit testing strategy is required to help catch low-level issues before being committed.

For example #252 is an issue that could have been caught in the integration phase.

Proposed Changes

1. Utilise MockSSH to simulate return SSH data. This will help mitigate issues that arise from untested data processing functions such as the TextFSM parser.
   2.A Functional testing workflow definition will improve our ability to catch issues that surface in production. A combination of Docker and something like TestInfra to assert network state might be a good idea.

[jathanism]

Related or dupe of #161