Remove usage of private '_url' property from OSC
Remove usage of private '_url' property from OSC

Due to a recent change in OSC (https://review.openstack.org/#/c/329189/)
some attributes that were previously private but available were removed.
This patch just assumes that the url is not available and gets the
endpoint regardless. It removes the necessecity for accessing the
private property.

Change-Id: I680f1a76aab1a94883bc58de241fd0d65d1cb78a
Closes-Bug: #1605876

openstack/python-mistralclient,StackStorm/python-mistralclient,openstack/python-mistralclient,StackStorm/python-mistralclient