Fix URL replacement in BaseOpenStackMockTest

This PR fixes the replacement of service URL strings containing spaces. How many of us have spent hours trying to debug something all because of a space? ;)
@demobox I can add the `:` back since it *is* more specific. I initially took the `matcher` approach, but just found that the simple regex worked great for the issue at hand. This is only going to be utilized by test classes that extend `BaseOpenStackMockTest`. Let me rework this a bit based on your feedback, thx!