Return request ID to callers

Currently, calls (create, get, etc.) return only the Image to a
caller. In order to log a mapping the request IDs of both glanceclient
and the caller, the x-openstack-request-id header value is needed on
the server side. This change allows that value to be bubbled up and
returned to the caller so that the appropriate logging can occur.
The return_req_id parameter can be set by services that are logging
request IDs. Glance's request ID will then be returned via the
return_req_id parameter.

This is a prerequisite for Log Request ID Mappings nova-spec to
be completed; Change Ib9b820a0feeb0c0e828ed3e4fab8261f8761ba9a

Change I43be05c351f901cee5509c76cff6d69f060c0b3f is an example of
a caller using this.

Implements: blueprint return-req-id
Change-Id: Ia82aa14db5f0e453010514fffb9a25d7b0fc2fd1
Merge "Return request ID to callers"