When using lioadm / targetcli to serve LVM volumes to instances
the former saves target configuration under /etc/target.
If cinder_volume container is upgraded / replaced then all targets
are gone and instances cannot connect to their volumes any longer.
This fix introduces target_config volume where corresponding
information is persisted.
Change-Id: I919bbe00853266037dd5c015810cc13c25e2e033
Closes-Bug: #1783569
acaebf83ce