Improve the default configuration for the writable user

By default, it now is:

writable.repository.address = vm://
writable.repository.user.name = siteuser
writable.repository.pool.name = writable
writable.repository.password = siteuser

I think siteuser cannot write. Imo, we should introduce a writable siteuser, which for example has read-access for the hst configuration and the same write access for documents as an 'editor'

Not completely sure however what the best default would be.

Others?