Show failure for Openstack Storate service_type

The openstack_service_type for the Openstack Storage Service (Swift) currently
defaults to "object-store".  However, if the Swift service is unvailable, the
authenticate_v2 method attempts to call:

    service_type.join ", "

The problem is that the String "object-store" doesn't have a join method.

The new tests show that "object-store" is an invalid service_type, whereas
["object-store"] is a valid service_type.

https://github.com/fog/fog/issues/3112

tests/openstack/authenticate_tests.rb

@@ -102,6 +102,26 @@ Shindo.tests('OpenStack | authenticate', ['openstack']) do
       end
     end
 
+    tests("v2 missing storage service") do
+      Excon.stub({ :method => 'POST', :path => "/v2.0/tokens" },
+                 { :status => 200, :body => Fog::JSON.encode(body) })
+
+      raises(NoMethodError,
+            "undefined method `join' for \"object-store\":String") do
+        Fog::OpenStack.authenticate_v2(
+          :openstack_auth_uri     => URI('http://example/v2.0/tokens'),
+          :openstack_tenant       => 'admin',
+          :openstack_service_type => 'object-store')
+      end
+      raises(Fog::Errors::NotFound,
+            "Could not find service object-store.  Have compute, image") do
+        Fog::OpenStack.authenticate_v2(
+          :openstack_auth_uri     => URI('http://example/v2.0/tokens'),
+          :openstack_tenant       => 'admin',
+          :openstack_service_type => %w[object-store])
+      end
+    end
+
     tests("v2 auth with two compute services") do
       body_clone = body.clone
       body_clone["access"]["serviceCatalog"] <<