Unit Tests for glance_http

This patch tests most of what is currently in use by Tempest of
glance_http.
There is a comment regarding this matter, but reafirming there is no
class testing against glance_http's https backend since it also ends up
using httplib.

Partially implements bp unit-tests
Change-Id: Iaa2477da4b559e4383fd8904ae4185edcb0dea36
Merge "Unit Tests for glance_http"