Extract create_nova_conf() from init_nova()

Allows creating a new nova.conf without wiping the database,
say, during an upgrade.

Change-Id: I7703b54bc3a32485b3cdab8d6185f93a6675040d
diff --git a/stack.sh b/stack.sh
index 118c2ef..35da935 100755
--- a/stack.sh
+++ b/stack.sh
@@ -1728,6 +1728,8 @@
 
 if is_service_enabled nova; then
     echo_summary "Configuring Nova"
+    # Rebuild the config file from scratch
+    create_nova_conf
     init_nova
 fi