deploy: allow not passing the env

This tries to read the wmcs-project file, if not there, then defaults
to prod to avoid deploying the dev environment in production if
there's any issue.

This is to adhere to the standard as it should be able to deploy
without needing any input arguments.

Bug: T316325
Signed-off-by: David Caro <dcaro@wikimedia.org>
Change-Id: I58e5638f1e46a6a12ba44b65b0e455dc838ab61a