make ceilometer work if you don't enable ceilometer-api

when doing ceilometer in a multihost devstack, you don't want
ceilometer-api running on the computes. Under -o errexit this
became fatal.

Change-Id: Ie43c8724ba467b810f5a3b075dea45d66dde8648
Merge "make ceilometer work if you don't enable ceilometer-api"