Switch heat template to use native nova resource

This change switches InstanceCfnInitTestJSON to use
the new resource type OS::Nova::Server instead of
AWS::EC2::Instance.

The next patch in this series will rename the file to refer to
"server" instead of "instance" as per nova's semantics

Change-Id: I1ae6460158a18517d430298602ad803fe1f8bb9f
Partial-Bug: #1187942
Merge "Switch heat template to use native nova resource"