Enable this option of 'terminate', which is mentioned in userconfig.yaml
and handled in _frontend/app.py:_handle_failure(). It appears to have
been left out of the valid_actions as an oversight.

Originally introduced in
2622d5da

Use a new helper function to simplify working with nodes that can only
accept certain strings. This will be used when adding the prompt.*
config options.

In later work we can see if this function would be useful elsewhere, and
could be added to '_yaml.py'.

Provide an option in buildstream.conf to disable the 'Would you like to
...' prompt when we cannot resolve a project.

Some users prefer not to be interrupted by such prompts, so pave the way
to creating options to disable all those that might get in the way.

Follow the example of the advice.* options 'git-config', and create a
namespace for these UI options grouped by behaviour, rather than an
over-reaching 'ui.*' namespace. In later work perhaps we'll also add
'advice.*' options.

Add a NEWS item for this.

Provide options in project.conf to disable the 'Are you sure ...'
prompts when making destructive changes:

- Add prompt.really-workspace-close-remove-dir
- Add prompt.really-workspace-reset-hard

Add a NEWS item for these.

Add 'prompt.*' config options to buildstream.conf

See merge request !887

The Refactor now makes it possible to test for the forth coming multi
element workspace open commands and to test for failure.

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.