Fabric: Let production() and staging() alter the settings

If production and staging commands can alter the settings based on the
received arguments it won't be necessary change all commands for altering the
settings before being run.

This way all the commands can be run in a feature staging server.
Fabric: Pass the branch once as argument when calling the environment

Set the branch in the settings when calling the environment instead of
passing it for every command which might need it.