Improving help text for Swift store opts.
Improving help text for Swift store opts.

Adding improved help text for swift_store_region, swift_store_endpoint
and swift_store_service_type

Partial-Bug: #1570946
Change-Id: I81cf3502c88dcaddb91ca8c20bd43e9c7790a72f

openstack/glance_store,openstack/glance_store