Add profiling support to keystoneclient

To be able to create profiling traces for Keystone, client should be
able to send special HTTP header that contains trace info.
This patch is as well important to be able to make cross project
traces. (Typical case nova calls keystone via python client, if
profiler is initialized in nova, keystone client will add extra
header, that will be parsed by special osprofiler middleware in keystone
api)

Don't worry no security issue here, trace information is signed by HMAC
key that is setted in api-paste.ini. So only person that knows HMAC key
is able to send proper header.

Change-Id: Ide6fe268613bb0cc9d9ec6fae2957cc570e9f851
Merge "Add profiling support to keystoneclient"