Allow deletion of multiple environments

Allow the CLI to delete multiple environments in one call,
as do the other openstack command line clients. Executes
sequentially, recording 404 errors but not treating them as
fatal.

Change has no unit tests partly because it's fairly trivial
but mostly because https://review.openstack.org/#/c/99177 needs
to land to add the framework.

Change-Id: I2d828c0ffe0a06b168c4642329c05fee616162dd
Merge "Allow deletion of multiple environments"