Returns a list of valid sub-keys.

def get_keys(self, key=None):
        """
            Returns a list of valid sub-keys.
        """
        if key:
            value = _lxc.Container.get_keys(self, key)
        else:
            value = _lxc.Container.get_keys(self)

        if value is False:
            return False
        elif value.endswith("\n"):
            return value.rstrip("\n").split("\n")
        else:
            return value