units: make sure container-getty@.service stops restarting when the pts device it is bound to is gone

We only want to restart the getty as long as the pts device is still
around. As soon as it is gone, the service should be removed to.

http://lists.freedesktop.org/archives/systemd-devel/2014-December/026048.html

units/container-getty@.service.m4.in

@@ -14,6 +14,7 @@ After=rc-local.service
 )m4_dnl
 Before=getty.target
 IgnoreOnIsolate=yes
+ConditionPathExists=/dev/pts/%I
 
 [Service]
 ExecStart=-/sbin/agetty --noclear --keep-baud pts/%I 115200,38400,9600 $TERM