Adopt glance.store library in Glance

This commits removes the old `store` package from glance and adopts the
usage of the new glance.store library. The library was designed to
preserve backwards compatibility as much as possible. In fact, most of
the changes in this patch are related to function args ordering and not
function renames or workflow changes.

Some changes that are worth mentioning:

1. Glance store doesn't rely on a global config object. All config
options must be explicitly registered.

2. All store operations now accepted an optional context. This is a
fallout from the context not being required in the `Store` constructor
anymore.

3. Store drivers are behind a private package called `_drivers` and
they're not suppose to be accessed directly. Instead, functions like
`get_store_from_scheme` should be used.

4. Stores are disabled by default

5. All the store specific options are under the `glance_store` group.

DocImpact:
The old store related configuration options have been moved under the
`glance_store` section. However, the old options will go through a
deprecation path. That is, they'll still be read from the `DEFAULT`
section to give deployers enough time to update their config files.

In k-2, the deprecated options will be completely obsolete.

Closes-bug: #1291848
Implements-blueprint: create-store-package

Change-Id: Iaacc70993ad5da292b93de42bbecda73d53b19fd
Merge "Adopt glance.store library in Glance"