Add a uri_join_paths helper function and tests

The standard URI::join method is not intended for joining URI relative
path segments. This function merely helps to accurately join supplied
paths. While the objective should be to use the OpenStack python
clients for much of the URL wrangling, there may still be times when
joining paths may be necessary.

All tests passing.

Change-Id: I39fe37b2b15ed05701d928065e6d70109634a382