add: possibility to have other backends in FakeNagiosEnvironment

This permits to drive FakeNagiosEnvironment with customised start/stop commands.
This looks a little unclean. Why not just provide actual start command instead of injecting magical {tmp_conf} in the start command string ?

If this is absolutely needed, consider either providing **kwargs to this method, or implementing FakeNagiosEnvironment.__getitem__() so that all instance variables can be used.