Do not parse Neutron error message to tolerate message changing

Two Neutron exception messages are being refined in
https://review.openstack.org/#/c/121767/. Need to use assertRaises instead of
assertRaisesRegexp to make test case tolerate message changing in the future.

Change-Id: Ia92f750c81b29ebe15e06d62a08b3f0ef14f3b84
Related-Bug: 1369871
1 file changed