Improve help text of http driver opts
Improve help text of http driver opts

Change-Id: Id5ee58fe2628039c70a8518a596fe86f62790939
Partial-Bug: #1570946
Co-Authored-By: Itisha Dewan <d5ff0d0370df5479b9a0e77a3d9cfed5954f2463@gmail.com>
Co-Authored-By: Hemanth Makkapati <f23e26dfb2093be0e64f803951bc9530a5ceae62@rackspace.com>
openstack/glance_store,openstack/glance_store