Fix availability zone client in compute admin test

In /tempest/api/compute/admin/test_availability_zone.py file,
V3 test class use the V2 Availability Zone client.

This patch correct the client for V2 and V3 by instantiating the
correct one in base class.

Change-Id: I97903b1ea7faa02c5880c091a166e386295de0da
Merge "Fix availability zone client in compute admin test"