Prescript plugin improvement

Added NetworkManager disabling mechanism. Merged prescript
and sshkeys plugins to prepare for fix of rhbz#976787.
This way ssh keys will be always distributed as the first
step.

Change-Id: Id9d708459d203bf5b2ce53478a57895928300f7d
Fixes: rhbz#967369, rhbz#976787
Merge "Prescript plugin improvement"