When install ceilometer with devstack, if do not set environment
variable OS_AUTH_URL, then devstack will set os_aut_url as empty
in /etc/ceilometer/ceilometer.conf, this will cause both
ceilometer-agent-central and ceilometer-agent-compute cannot start.
Fix bug 1205776
Change-Id: I32c77c92dc2e9d03c86e703f170e0216dd829a61