Update icinga install script to enable icmp checks

This patchset updates the icinga-core installation script to
setup the check_icmp icinga plugin as setuid.

For some reason the package doesn't seem to do this so we need to
do that here.

I've also tidied up the set -eux and set -pipefail at the top of
the script.

Change-Id: I35f5e3ffcfcab5d93a7bf902ba12fc7d0afea691
Merge "Update icinga install script to enable icmp checks"