Rename private methods of glance_http

These methods are used in glance_http module only, and we will
migrate the module to tempest-lib with images_client together.
So it is necessary to clarify these methods as private ones.

Partially implements blueprint consistent-service-method-names

Change-Id: Id27a99ed9f6973c67d7df0601b5fae40ca562de7
2 files changed