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