Declare a urlPrefix for reuse in NorthboundIT.

All test cases use the same url prefix, but prior to this patch
it was copy pasted in all test methods.
For easier code maintenance, declared it once in the
class and reused it in all test cases.

Change-Id: I4c6430799c935293c7b9715b5b68e84e4baefd3a
Signed-off-by: Alissa Bonas <abonas@redhat.com>