Start nova-compute with child cell conf

A recent commit broke the cells support and switched nova-compute to always
start with the API cell .conf.  This corrects the regression.

Change-Id: I633344c8784c154f61e751cd0a408196e61525b3
Closes-bug: 1243961
diff --git a/lib/nova b/lib/nova
index 809f8e5..31f286d 100644
--- a/lib/nova
+++ b/lib/nova
@@ -613,21 +613,25 @@
 
 # start_nova_compute() - Start the compute process
 function start_nova_compute() {
-    NOVA_CONF_BOTTOM=$NOVA_CONF
+    if is_service_enabled n-cell; then
+        local compute_cell_conf=$NOVA_CELLS_CONF
+    else
+        local compute_cell_conf=$NOVA_CONF
+    fi
 
     if [[ "$VIRT_DRIVER" = 'libvirt' ]]; then
         # The group **$LIBVIRT_GROUP** is added to the current user in this script.
         # Use 'sg' to execute nova-compute as a member of the **$LIBVIRT_GROUP** group.
-        screen_it n-cpu "cd $NOVA_DIR && sg $LIBVIRT_GROUP '$NOVA_BIN_DIR/nova-compute --config-file $NOVA_CONF_BOTTOM'"
+        screen_it n-cpu "cd $NOVA_DIR && sg $LIBVIRT_GROUP '$NOVA_BIN_DIR/nova-compute --config-file $compute_cell_conf'"
     elif [[ "$VIRT_DRIVER" = 'fake' ]]; then
         for i in `seq 1 $NUMBER_FAKE_NOVA_COMPUTE`; do
-            screen_it n-cpu "cd $NOVA_DIR && $NOVA_BIN_DIR/nova-compute --config-file $NOVA_CONF_BOTTOM"
+            screen_it n-cpu "cd $NOVA_DIR && $NOVA_BIN_DIR/nova-compute --config-file $compute_cell_conf"
         done
     else
         if is_service_enabled n-cpu && [[ -r $NOVA_PLUGINS/hypervisor-$VIRT_DRIVER ]]; then
             start_nova_hypervisor
         fi
-        screen_it n-cpu "cd $NOVA_DIR && $NOVA_BIN_DIR/nova-compute --config-file $NOVA_CONF_BOTTOM"
+        screen_it n-cpu "cd $NOVA_DIR && $NOVA_BIN_DIR/nova-compute --config-file $compute_cell_conf"
     fi
 }