Retrieve user_id from HTTP Headers to populate Context

We need the user_id with notifications sent by Heat to be able to
handle it in Ceilometer, currently only the username is provided which
can be non-uniq per tenant and could snip the storage unity.
The easiest way to do that is to populate the Context with the user_id
retrieved from Headers.

Related-Blueprint: send-notification

Change-Id: I88bd5612556d2000d72427308d17643f14d35306