Make salt-cloud interaction more API accessible.

* Created `SaltCloudException` and `SaltCloudSystemExit` in `saltcloud.exceptions`. Any exception salt-cloud needs to raise should be a subclass of `SaltCloudException`. `SaltCloudSystemExit` is used to trigger a system exit call which should only exit if being executed from the console.
* Cleaned up some unnecessary imports.
* Fixed some unnecessary logging calls just to show a traceback in debug level.