[openstack] Ensure password sent is of type string

Signed-off-by: Nelvin Driz <nelvindriz@live.com>

lib/fog/openstack.rb

@@ -92,7 +92,7 @@ def self.authenticate_v2(options, connection_options = {})
           'auth' => {
             'passwordCredentials'  => {
               'username' => @openstack_username,
-              'password' => @openstack_api_key
+              'password' => @openstack_api_key.to_s
             }
           }
         }