Retrieve user_id from HTTP Headers to populate Context

We need the user_id within 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 confuse the integrity of data.
The easiest way to do that is to populate the Context within the user_id
retrieved from Headers.
No need to store the user_id in Heat backend.

Related-Blueprint: send-notification

Change-Id: I88bd5612556d2000d72427308d17643f14d35306