Added support for setting global_request_id on a Connection.
If done, this will cause all requests sent to send the request id
header to the OpenStack services. Since Connection can otherwise
be used multi-threaded, add a method global_request that returns
a new Connection based on the old Connection but on which
the new global_request_id has been set. Since a Connection
can be used as a context manager, this also means the global_request
method can be used in with statements.
Change-Id: I70964cdd79741703c0b9b911b3b2f27c248130f0
95bf14908e