WIP: Set ArtifactCache for push/pull correctly when `--remote` is used

Description

This fixes the bug in #798 so user will be able to specify --remote for their bst pull operations and initialization won't fail due to Push not allowed errors - because that was never asked for.

Changes proposed in this merge request:

• Similar to artifact_remote_url, add artifact_remote_can_push to Stream._load() and use that in ArtifactCache.setup_remotes(). See first commit.
• Refactor the codepath, starting from second commit: This hopefully paves the way for multiple usages --remote and making the future fix for bst push --remote (--cert etc. needs to be added to be actually usable) less hacky.

This merge request, when approved, will close: #798

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