Forbid availability_zone None in aggregate Nova API

In aggregate API, argument "availability_zone" became optional in the request.
Fixed tempest code same as nova.

Change-Id: I73f42c6c069a9bc6318ac3bac5ec94b61c65ecaf
Related-bug: #1252177
Correctly call client inits from test_multi_backend

This commit fixes the client init calls in setUpClass for
test_mult_backend. Since this doesn't run in the gate it slipped through
the massive config refactor.

Closes-Bug: #1276215

Change-Id: Ibcd521a5b5c3f4d755387b29ee73dcc042c44fa0
Merge "Forbid availability_zone None in aggregate Nova API"
Merge "Correctly call client inits from test_multi_backend"
Merge branch 'master' of https://github.com/openstack/tempest