WIP: Allow running bst commands from workspace directory

Description

This patch will allow users to run BuildStream commands from workspace directories. Things left to do:

• Decide what happens when more than one project are using the same workspace
• Make bst build (without any arguments) work from the workspace directory
• Add tests

Changes proposed in this merge request:

• Allow running bst commands from worksapce directory

This merge request, when approved, will close issue/bug: #222 (closed)

CHANGELOG/Release Notes

---