Port extended-server-attributes API into v3 part2

Ports the extended-server-attributes extension and the corresponding
unittests to the v3 framework.

Partially implements blueprint nova-v3-api

Change-Id: I33e227de0e4d3e1e5f52ca79157e3846a0e6ee13
Merge "Port extended-server-attributes API into v3 part2"