Deprecate primary_network_interface config parameter

[BytePaws]

Proposed Changes

Remove the requirement for the user to specify a network interface as primary.
A primary network interface should be automatically identified by being:

1. Up
2. Connected
3. Not set to management only?

Justification

The primary_network_interface config parameter - which is not documented since rewriting the config_parser module with #83 - is currently required for payload translation.
An effort should be made to patch that field for the specific device after registering all of its interfaces.

Impact

The impact for users would be minimal. They would not be required to decide which of the devices interfaces (which can fluctuate in names or number) should be primary.