Convert multinic v3 plugin to v2.1

Changes required to have v3 plugin natively support
the v2.1 API.

add_fixed_ip -> addFixedIp
remove_fixed_ip -> removeFixedIp

Removes v3 version of the multinic test and changes to
share use of the v2 unittests code. Backports a v2.1 specific
test.

Partially implements blueprint v2-on-v3-api

Change-Id: Ie01eddf1e772092c291b1475404f87e6e6f47c71
Merge "Convert multinic v3 plugin to v2.1"