Implemented connection pooling in vrtManager/connection.py:
  * connections are shared among threads (not processes)
  * settings LIBVIRT_KEEPALIVE_INTERVAL and LIBVIRT_KEEPALIVE_COUNT added to control the clientside keepalive protocol
  * one connection per host will be created and used as long as it is alive
add new settings and explanation to settings.py and local_settings.py.example
remove socket host active check as it leads to weird messages in auth.log (at least for ssh connections). Connection error messages should be enough to determine what went wrong.
In the future it may also be possible to set "connection failed" based on the last error code returned by libvirt.