Bug#1304126

Possible fix for Bug #1304126

In the SetEnvironmentCommand's Init function I added an additional API call to retrieve existing environment config. All keys a user specifies in the "juju set-env" command are checked against the existing config keys and if the user defined key does not exist in the existing config a warning of a possible misspelling is issued.

This way if the users tries: 
juju set-env ss-hostname-verification=false 

A warning will be returned (though the setting is still forwarded to the state):
2014-10-23 09:45:03 WARNING juju.cmd.juju environment.go:135 key ss-hostname-verification is not defined in the current environemnt configuration: possible misspelling


In addition i fixes a few capitalisation in some of the returned error messages.