Code dedup in tests for server._action_rebuild

Externed common parts to setUp():
- request body
- request object
- test image UUID
- test image URL
These parts were equal in the most part of the methods.

bp nova-tests-code-duplication

Change-Id: Ie29dd090075915a4065ee395b4e50fc178f72f3b
Merge "Code dedup in tests for server._action_rebuild"