Each time we call that script, we must set a few env vars
beforehand. Let's make these vars optional parameters to
the script instead.
Also add helper scripts to list the locations (zones or
regions) available to each provider.
GCP quotas are fairly limited (on my account, I can only
use 8 public IP addresses per zone, which means that I cannot
deploy many public clusters in a single zone). I tried to
use private clusters, but that causes other problems.
This refactoring makes it possible to spread clusters
across multiple zones. Since I have access to 20+ zones
in Europe and 20+ zones in the US, this lets me create a
lot of public clusters and simplifies the module quite a bit.
