Bug#887563: corosync prerm will stop pacemaker and not start it again

[Anders Kaseorg]

Control: found 887563 2.4.2-3
Control: severity 887563 important

This just bit me on a Stretch cluster when upgrading corosync from 2.4.2-3 
to 2.4.2-3+deb9u1.  Marking as such.  Please apply the suggested fixes as 
soon as possible.

Anders