Port fixed_ip extention to v2.1

This patch ports fixed_ip 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: Ifa7e0dd5a91cfe881077b92aeb7019d0435d780a
Merge "Port fixed_ip extention to v2.1"