Install python-libvirt for ceilometer only if n-cpu enabled

Nova is responsible for installing the libvirt package (if it is being
used). It is required by python-libvirt but python-libvirt only required
in ceilometer if nova compute is being used. There are some usage
scenarios where nova compute is not being used so in that case don't
install python-libvirt.

Change-Id: I0db66f1c0526e24ade98de85989a5ed8d37f0c4f

lib/ceilometer

@@ -334,6 +334,8 @@ function install_ceilometer {
         pip_install_gr pymongo
     fi
 
+    # Only install virt drivers if we're running nova compute
+    if is_service_enabled n-cpu ; then
         if [[ "$VIRT_DRIVER" = 'libvirt' ]]; then
             pip_install_gr libvirt-python
         fi
@@ -341,6 +343,7 @@ function install_ceilometer {
         if [[ "$VIRT_DRIVER" = 'vsphere' ]]; then
             pip_instal_gr oslo.vmware
         fi
+    fi
 
     if [ "$CEILOMETER_BACKEND" = 'es' ] ; then
         ${TOP_DIR}/pkg/elasticsearch.sh download