Code dedup in class ServersControllerCreateTest

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

bp nova-tests-code-duplication

Change-Id: I091f14a9753059d759ba2f7b05ff33f5abea0f00
Merge "Code dedup in class ServersControllerCreateTest"