Plugin request helpers

There are a number of growing de-facto standards around the ways that services should talk to each other. For the moment those i know of are:
 - You should include the X-Service-Token when making requests between services
 - You should include the X-OpenStack-Request-id from the incoming request in outgoing requests.

And i'm sure others will come along.

Given that we pass an auth plugin down from keystonemiddleware that correctly fills the X-Auth-Token and that the above information is generally available from auth_token middleware the plugin that is presented to the user should transparently do 'the right thing' as much as possible to distance these standards from service writers.