THOTH_CEPH_PREFIX should use the same mechanims as other env vars

[VannTen]

Problem statement

As a Thoth developer, I would like to have a single place defining ceph prefix in CephStore, using and environment variable. (Either in storages code itself, or in other components using storage)

Proposal description

Use same logic that for bucket and other (see thoth/storages/ceph.py). I think this needs some work on adapting storages codes which set the prefix itself for "subprefixing".

Introducing a global prefix / sub prefix might be something to think about

Additional context

Relevant: #2700

Acceptance Criteria

• [ ] Only CephStore use THOTH_CEPH_PREFIX in storages, and CephStore().prefix == os.getenv('THOTH_CEPH_PREFIX')

/kind feature /sig stack-guidance

[VannTen]

Same remarks for THOTH_DEPLOYMENT_NAME

[VannTen]

/priority important-longterm