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
5 files changed