LVM-activate: Stop before blk-availability.service

If storage services (e.g., iscsi-shutdown.service) stop before an
LVM-activate resource stops, the managed VG may become unavailable. Then
the LVM-activate resource may fail to deactivate the volume group and
thus fail its stop operation.

This commit adds a systemd drop-in "After=blk-availability.service"
directive for resource-agents-deps.target during the LVM-activate start
op. blk-availability includes "After=" directives for other storage
services and thus serves as a convenient wrapper.

blk-availability is not enabled by default, and a "Wants=" drop-in
that's created after Pacemaker starts would not be able to start
blk-availability automatically. So here we also start blk-availability
during LVM_start().

Resolves RHBZ#1902208

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>