Update the add machine api to accept both environment name and UUID.
Add a method to get the environment uuid from the client api connection.
Use the environment uuid instead of the name.
Merge pull request #249 from howbazaar/env-cmd-add-machine

Update add machine to use environment UUID

This is the start of the work to hide the EnvName from the environment command base.

This will allow us to specify an environment using either other command line parameters or environment variables to better allow scripts to call juju without having a $(JUJU_HOME)/environments directory.