Call super class after extension check in vpnaas api test

This commit fixes a bug in test_vpnaas_extensions.py. If the extension
is disabled then the tenant isolation code is called generating a new
tenant and the resources needed to use it in this test. However, since
the skip exception is raised after the super() call the tearDownClass
will not be run. This will cause the resources created by tenant
isolation to never get deleted. This commit fixes this behavior by
moving the skip before the super call.

Change-Id: Id7915978e7a42f641ff99493cb3918424dd98674
Closes-Bug: #1268852
Removes leftover self.conf from isolated creds

Partially implements bp config-cleanup

Change-Id: I069fd68eae3998c62efd144e1d5a44ca185e3511
Merge "Removes leftover self.conf from isolated creds"
Merge "Call super class after extension check in vpnaas api test"