Change v3 admin-password to v2.1

This patch changes v3 admin-password API to v2.1 and backport v3
unit test cases to v2.1.
The differences between v2 and v3 are described on the wiki page
https://wiki.openstack.org/wiki/NovaAPIv2tov3

Partially implements blueprint v2-on-v3-api
Change-Id: I167753eddd188120236c51bade549c907c8a8466
Merge "Change v3 admin-password to v2.1"