Drop privileges to the salt-master user if need be. Fixes #726.

As soon as salt-cloud starts, either through CLI, or via API, if running as root, salt-cloud drops privileges to the salt-master root. If unable exits with a warning to what just happened.
If from CLI, call `self.error()`.
Merge pull request #730 from s0undt3ch/issues/726-drop-privileges

Drop privileges to the salt-master user if need be. Fixes #726.