Allow backup/ failover regions to be specified

[jimburtoft]

In the experiments, allow the user to specify region:
for example:
us-east-1 | us-west-2
Treat this as a config which says try us-east-1 first but then failover to us-west-2 if the deployment fails.

Deployments could fail because of lack of quota or insufficient capacity.

However, if the deployment in the first region succeeds, I do NOT want to deploy into the second region.

(this should also flow through from fmbench orchestrator)