Mark bst checkout/pull/push as obsolete

Background

See https://mail.gnome.org/archives/buildstream-list/2018-November/msg00106.html and its followups for full context.

As we aim to provide bst artifact group of commands to do all operations related to artifacts, we should fold the existing commands that do similar things in that group. So, in addition to providing new commands like log, list etc, we should also look into moving pull, push and checkout under that group (similar to how we did so for the bst source group).

Task description

• Provide new functionality under bst artifact group - See WIP !1008 (closed).
• Mark bst chekcout as obsolete (this should be done in the same way as we handle fetch and track).
• Move pull and push under bst artifact group, and mark them as obsolete.

Acceptance Criteria

All functionality that requires dealing with artifacts, should be moved under bst artifact command group, and existing alternatives should be marked as obsolete.

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