Replicator: extend volume_group with list of VGs and flag

Replicator: extend volume_group with list of VGs and flag

Add pointer to linked list of opened VGs. List temporarily keeps
the information about needed or locked and opened VGs for replicator target.

Also add cmd_missing_vgs flag information for quick check and
also for possible continuos process_each_lv() usage where we need
to detect whether failure has been caused by missing VG or
some other reason.