Port floating_ip_dns extention to v2.1

This patch ports floating_ip_dns 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: Ia1316697c141fde2b431ba79aebae5986687a4fa
Merge "Port floating_ip_dns extention to v2.1"