Add a default available for all subsites available configuration part

Forge plugins that want to bootstrap some hst configuration cannot do this now, as the path where it should be added contains the project name: For example

hst:hst/hst:configurations/myproject/hst:components/....

See the myproject part.

There for, we will add a default available part:

hst:hst/hst:configurations/hstdefault.

All configuration below hstdefault is available to every (sub)project. Projects can 'hide' hstdefault configuration by configuring the same nodename configuration themselves again.