Fix missing NovaComputeLibvirtType parameter.

Also change default to '' as null is not technically allowed as a
string default in cloudformation (Heat accidentally allows this).
Os-apply-config, via pystache, will still treat this as false for
the purposes of checking if it is set or not.

Change-Id: Ia02dbcf619bdc92647f1d21157fa4a8e3f749de3

nova-compute-instance.yaml

@@ -31,7 +31,7 @@ Parameters:
     Default: libvirt.LibvirtDriver
   NovaComputeLibvirtType:
     Type: String
-    Default: null
+    Default: ''
   NovaApiHost:
     Type: String
   GlanceHost:

overcloud-source.yaml

@@ -13,6 +13,9 @@ Parameters:
   NovaComputeDriver:
     Default: libvirt.LibvirtDriver
     Type: String
+  NovaComputeLibvirtType:
+    Default: ''
+    Type: String
   NovaImage:
     Type: String
     Default: overcloud-compute