Port floating_ips extension to v2.1

This patch ports floating_ips extension from v2 to v2.1,
and have v2 unit test cases shared between v2.1 and v2.

Partially implements blueprint v2-on-v3-api

Change-Id: Ib06c0d6c7ff123f09a99ed94f4b0009695897602
Merge "Port floating_ips extension to v2.1"