Add password recovery procedure for default admin user

ISSUE:
  Need for a password recovery procedure for the controller default network administartor user (admin)
  which can be performed only by a user which has access to the host machine file system
PROCEDURE:
  1) System admin creates a flag file in startup directory
  2) Restarts User Manager implementation bundle (or the whole controller) through osgi console

  The above two actions will reset the default network administrator password to the factory one, "admin".
  Will log the change to console and remove the flag file.

Change-Id: Iba5cfd908ef311beeb74c6b11df3bbc6b34872cb
Signed-off-by: Alessandro Boch <aboch@cisco.com>