Make lio iSCSI changes persistent to avoid lost

Make lio iSCSI changes persistent to avoid lost

To avoid losing iSCSI configuration when target.service is
started/restarted we make persistent any change Cinder makes to the
configuration.

This will allow us to coordinate access from multiple rtslib users
(target daemon, targetcli, cinder...)

This patch changes rtstool and adds an additional command "save" with
optional parameter the filename where we want to save current
configuration. If no parameter is provided it saves to default location
defined by rtslib.

Closes-Bug: #1427301
Change-Id: I74bd09ed16a2e0e584d3e04762aec8cbdf101f6d