Build & deployment improvements

Add support for graceful restarts (disable gear in web proxy during
deploy, re-enable after deploy)

Redirect socat output to /dev/null to avoid printing a blank line to
the client
Build & deployment improvements

Modifications to rollback:
- Ability to go back to a specific deployment id (if specified),
or the previous deployment if not.
- Don't delete deployments any more
- Don't stop the gear any more (the internal call to activate will do
that)