add enable/disable actions for targets
Description
We need to have an action for enabling/disabling iSCSI targets.
Proposal
Disabling
As targets are providers, we should check if there are any active sessions and if there is any, we should fail - we must be sure that none of our clients use our volumes. If there are no sessions, just stop and disable systemd service.
Enabling
Enabling is trivial - just enable and start service.