Host: add driver option for manual setting driver
Protocol: accept driver string in constructor

Init manual_driver with None and try to set it, if there driver
parameter is given.
If invalid driver parameter is given, we ignore it and write a debug
message.
Queue: host_driver parameter for all hosts

Introduce host_driver parameter, that sets driver manually for all
hosts.
Merge pull request #64 from knipknap/feature/manual_driver

setting driver manually