Rethinking our filesystem access

In Rocky multiple backend support is added as experimental feature. In order to take advantage of this feature it is decided to deprecate work_dir and node_staging_uri configuration options and reserve two filesystem stores 'os_glance_tasks_store' and 'os_glance_staging_store', which can be used to get rid of initializing store via internal functions.

These stores will be ``hard-coded`` in glance for the time being and injected to enabled_backends config option to load when
glance_store will be initialized at the time of glance-api service starts. Operator needs to insure that these stores will not be
included in enabled_backends config option in glance-api.conf.