Idempotency for LXC cloning

Previously, clone operations where the new container already exists were reporting as failed. This commit modifies that behavior so that if the new container exists, it returns the facts for that container instead of failing.

To facilitate these changes, a `_container_exists` method was added which returns True or False based on whether or not the container name is in the output from `self._list()`.

I also made a few fixes in the documentation by removing trailing spaces and fixing a typo in the state on line 222.