Port migrations extension to v3 API part 1

This is part 1 of the two part commit to port migrations to v3.
As part of this commit the api and test files are copied over to v3
plugins folder. The second changeset will have changes to port
migrations to v3.

Partially implements blueprint nova-v3-api

Change-Id: I94d4ce6903da4c16545092f5618c4d130b700462
Port migrations extension to v3 API part 2

This patch contains changes to port migrations extension to v3.

Partially implements blueprint nova-v3-api

Change-Id: Ie79a0d1eeeee7d4fce7a61d8e0cec90982368cf2
Merge remote-tracking branch 'm_src_repo/citrix-fixes' into build-temp