Fix folder creation at quickstart

mkdir to create /etc/kolla/config/nova fails if config
folder is not created.

This change adds `-p` to create both config and nova folders

Change-Id: Ibceaa437834cbba681b30c418da3c15d75d49945

doc/quickstart.rst

@@ -608,7 +608,7 @@ the content shown below.
 
 ::
 
-    mkdir /etc/kolla/config/nova
+    mkdir -p /etc/kolla/config/nova
     cat << EOF > /etc/kolla/config/nova/nova-compute.conf
     [libvirt]
     virt_type = qemu