Create a unified request identifier

Create a unified request identifier associated with an API request that crosses OpenStack service boundaries. This will enable more effective debugging of requests that span multiple services. For example, a Nova instance create request may touch other services including Glance and Quantum. A single id will simplify the process of tracking down errors.

See related email discussion: https://lists.launchpad.net/openstack/msg13082.html

Proposal:
-The first OpenStack service to touch a request will tag it with an "OpenStack request id". This will effectively be a global identifier. Inter-service requests will contain this ID in an HTTP header, "x-openstack-request-id". This value will be a UUID.

-Each service that touches the request will carry along this header value and use it in log messages and external notifications.

-Each service may have its own internal request identifier (request_id in Nova and Glance) and should also log this value in conjunction with the global identifier.

1. There shouldn't be a cross-service request ID, but instead a mapping of the request IDs from the calling service to the answering service.

2. The log message containing the mapping will be in the n-api log.

3. The x-openstack-request-id header will be used across projects; for nova (and cinder), x-compute-request-id will be deprecated in the future.

4. Since nova will be the one logging the mapping, the clients will need to provide the x-openstack-request-id value back with its response.

5. Since this will be rolled-out across services, common components should be shared in oslo.