Correct name of Ceilometer central agent service for Fedora and EL

Fedora and EL-based operating systems use the name
"openstack-ceilometer-central" for this service rather than
"openstack-ceilometer-agent-central.

As such while there is overlap with the instructions for openSUSE and
SLES this slight difference needs to be accounted for.

Change-Id: I8f922cafd35e1eedfa8d00f14cf07acbeefe842f
backport: havana
Closes-Bug: #1252265
Merge "Correct name of Ceilometer central agent service for Fedora and EL"