[openstack] Ensure String Username for Authentication

When username is composed of only integers, openstack raises a
ProgrammingError when trying to authenticate user.

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

lib/fog/openstack.rb

@@ -95,7 +95,7 @@ module Fog
         req_body = {
           'auth' => {
             'passwordCredentials'  => {
-              'username' => @openstack_username,
+              'username' => @openstack_username.to_s,
               'password' => @openstack_api_key.to_s
             }
           }