Update workspace CLI including allowing default locations

Background

Updating the workspace cli including default workspace locations.

This will replace bolted issues inc #229 (closed)

The final proposal is on the ML https://mail.gnome.org/archives/buildstream-list/2018-September/msg00046.html and this issue and MR's should focus on implementing what was agreed on the ML.

Task description

• Add default workspace locations
• Update workspace CLI

Acceptance Criteria

When the CLI changes agreed on the ML are implemented.

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