ML2 Cisco Nexus MD: Support portchannel interfaces

Port of port-channel interface support that was implemented for the cisco
core plugin - https://review.openstack.org/#/c/42037

Note that additional port-channel UT already existed. (Created during
initial cisco core nexus subplugin -> ml2 cisco nexus md port.)
See test_cisco_nexus.py module, NEXUS_PORTCHANNELS references.

Closes-Bug: 1294900
Change-Id: Ifc64f605e5783ee1e85d66c87f422f64b47fa996