Feature Request: allow freezing/thawing an environment

[Clint Byrum]

As I'm setting up a production ensemble system, it struck me how easily I could screw it up given a few careless commands.

It would be a useful feature to be able to freeze an environment with a message, something like

$ ensemble freeze -m "Production Configuration! Use Caution."

And then whenever you do anything that may modify the environment you get

$ ensemble destroy-service jenkins-master
2011-08-24 16:18:28,393 INFO Environment 'production' was frozen at 2011-08-24 16:01:33,845 with message:
2011-08-24 16:18:28,393 INFO "Production Configuration! Use Caution."
2011-08-24 16:18:28,393 INFO Thaw environment to enable changes.
$

This would be especially useful for revision control of the environment's configuration, as you could simply wrap all revisions in thaw/freeze and the messages would be something like 'Frozen after revision 495'.

As an alternative or maybe even better way to implement this, one could also use a lock/unlock/break-lock model to have one controlling CLI at a time.