systemd/ceph-disk: make it possible to customize timeout

When booting a server with 20+ HDDs udev has to process a *lot* of
events (especially if dm-crypt is used), and 2 minutes might be not
enough for that. Make it possible to override the timeout (via systemd
drop-in files), and use a longer timeout (5 minutes) by default.

Fixes: http://tracker.ceph.com/issues/18740
Signed-off-by: Alexey Sheplyakov <asheplyakov@mirantis.com>
(cherry picked from commit 22332f6)

systemd/ceph-disk@.service

@@ -6,5 +6,6 @@ Wants=local-fs.target
 [Service]
 Type=oneshot
 KillMode=none
-ExecStart=/bin/sh -c 'timeout 120 flock /var/lock/ceph-disk-$(basename %f) /usr/sbin/ceph-disk --verbose --log-stdout trigger --sync %f'
+Environment=CEPH_DISK_TIMEOUT=300
+ExecStart=/bin/sh -c 'timeout $CEPH_DISK_TIMEOUT flock /var/lock/ceph-disk-$(basename %f) /usr/sbin/ceph-disk --verbose --log-stdout trigger --sync %f'
 TimeoutSec=0