Front/reat port connection always shows B side of connected cable

[jeremystretch]

Environment

• Python version: 3.5.2
• NetBox version: 2.5.5

Steps to Reproduce

1. Create or find a front or rear pass-through port with a connected cable.

Expected Behavior

The "front ports" and "rear ports" tables on the device view should list the far end of each attached cable.

Observed Behavior

The B side is always listed. This is due to a bug in the get_cable_peer() method:

def get_cable_peer(self):
    if self.cable is None:
        return None
    if self._cabled_as_a:
        return self.cable.termination_b
    if self._cabled_as_b:
        return self.cable.termination_a

The _cabled_as_x methods each return a generic relation manager, whereas what we need is the queryset.

[tvberlin]

@jeremystretch thanks for implementing!