Initialize sec to None to avoid exception in security group

When an existing security group is not found in nova, heat will trigger
an exception and the stack won't be created.

Signed-off-by: Steven Dake <sdake@redhat.com>

heat/engine/resources.py

@@ -211,6 +211,7 @@ class SecurityGroup(Resource):
             return
         self.state_set(self.CREATE_IN_PROGRESS)
         Resource.create(self)
+        sec = None
 
         groups = self.nova().security_groups.list()
         for group in groups: