systemd: various updates and fixes

systemd: various updates and fixes

Don't use lvmetad in lvm2-monitor.service ExecStop to avoid a systemd issue.
 - a systemd design issue while processing dependencies
   with socket-based activation that ends up with a hang
 - https://bugzilla.redhat.com/show_bug.cgi?id=843587
   (also tracker bug https://bugzilla.redhat.com/show_bug.cgi?id=871527)
 - not using lvmetad in this case is just a workaround, once the bug
   above is resolved, we should enable the lvmetad in that specific case

Remove dependency on fedora-storage-init.service in lvm2 systemd units.
 - fedora-storage-init.service and fedora-storage-init-late.service is
   going to be separated into respective units that belong to each block
   device subsystem:
     - mpath + mdraid activated via udev solely
     - dmraid with its own dmraid-activation.service unit
     - lvm2 with the lvm2-activation-generator to generate the
       activation units runtime if lvmetad disabled
       (global/use_lvmetad=0 set in lvm.conf) and activation done
       via udev+lvmetad if lvmetad enabled (global/use_lvmetad=1 set
       in lvm.conf)

Depend on lvm2-lvmetad.socket in lvm2-monitor.service systemd unit.
 - as lvm2-monitor uses lvmetad if lvmetad is enabled