Configure the ec2 zone explicitly

Tempest now creates Availability zones which in parallel run,
may causes issues with EC2 tests.

The EC2 tests will use the preconfigured zone, instead of just picking a
random existing zone.

Closes-Bug: 1265672
Change-Id: I12dfaca868a3f0aec2a85fe45d8a524940d9eae0
Merge "Configure the ec2 zone explicitly"