Glance api to pass identity headers to registry v1

This patch introduces the send_identity_headers
config option that allows glance-api to pass auth identity
headers when making calls to the registry v1.

docImpact
Fixes bug 1199990

Change-Id: Ie5f07ed6dfeaa8428de4f79c4d40d182328e6ab4
Merge "Glance api to pass identity headers to registry v1"