Slighty refactored url_prefix usage in WSGI tests

Several calls to url_prefix has been replaced
with the usage of a common variable in WSGI tests
in order to adjust to DRY principles.

Change-Id: I056e1e8879a97f6a30aff924e55bdf50897c08e4
Closes-Bug: #1285219
Merge "Slighty refactored url_prefix usage in WSGI tests"