Port floating_ips_bulk extention to v2.1

This patch ports floating_ips_bulk extention 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: I9444ff9c394d530cb76dd4ac21b51cd4de7ef6dc
Merge "Port floating_ips_bulk extention to v2.1"