Workspace CLI update

This is to update the workspace CLI to as agreed on the mailing list
https://mail.gnome.org/archives/buildstream-list/2018-September/msg00046.html

This patch also introduces the default workspace directory.

tests/frontend/workspace.py: Refactor of support functions.

Refactoring the support functions for the workspace cli tests.

The main motivation for this was to allow multiple elements to be
created before running the workspace command.

Also the support functions are now broken up so that test that test for
failure can still reuse all the function that create elements, but call
bst workspace with custom arguments and expect other things than success.