bug #1777: case for unknown EC2 SignatureVersion

Patch provided by Piotr Sawicki <p.sawicki@task.gda.pl>

src/cloud/common/CloudAuth/EC2CloudAuth.rb

@@ -23,6 +23,7 @@ module EC2CloudAuth
         signature = case params['SignatureVersion']
             when "1" then signature_v1(params.clone,one_pass)
             when "2" then signature_v2(params.clone,one_pass,env,true,false)
+            else return nil
         end
 
         if params['Signature'] == signature