Add unit tests for image waiter

This commit adds unit tests for the image waiter in
tempest.common.waiters. It adds tests for both the timeout case and
the success path.

Partially implements bp unit-tests

Co-Authored-With: Sean Dague <sean.dague@samsung.com>

Change-Id: Ib0501cd3bc323fd036444dacf884879963842e50
Merge "Add unit tests for image waiter"