Adds migrate server extension for V3 API

Moves the migrate/live-migrate server functionality out of admin_actions into
its own extension. This part of the blueprint v3-admin-actions-split
allows more selective enablement of features contained in the admin
actions extension.

Note the XML api samples are no longer generated because
bp remove-v3-xml-api has been approved.

Refactor removes some exception handling for migrate along with the relevant tests
as those exceptions will never occur.

Partially implements bp v3-api-admin-actions-split
DocImpact: Adds os-migrate-server extension and moves migrate/live-migrate
functionality out of os-admin-actions into this new extension.

Change-Id: I6b29f501ad6bb9a6401b1c20cd419d9e05fe369b
Merge "Adds migrate server extension for V3 API"