Part of a fix for !1124

This means that keys are only generated after tracking.

Part of a fix for !1124

Tweak cache-buildtrees option

See merge request BuildStream/buildstream!1208

Caching buildtrees may significantly increase CAS disk usage. Change the
default to cache buildtrees only when required for core functionality.

We anticipate other cases than build failures where buildtree caching
will be required.  E.g., incremental workspace build with remote
execution. Or running tests in a buildtree in parallel with the build of
reverse dependencies.

This renames the option value 'failure' to the more generic 'auto' to
cover these other cases as well.

In the interactive failure handler we should never enter the interactive
shell if the buildtree is missing (due to cache-buildtrees=never).

Creating an empty buildtree directory when buildtree caching is disabled
means cached_buildtree() can return True even if no buildtree is
available, which breaks logic in the frontend and in Stream.

This adds the buildtree_exists() method to indicate whether an artifact
was created with a buildtree, independent of whether the buildtree is
available in the local cache.

Return False instead of raising an error.

This removes extraneous spaces.