taskserver: Allow helper tool in manual config

The helper tool so far was only intended for use in automatic PKI
handling, but it also is very useful if you have an existing CA.

One of the main advantages is that you don't need to specify the data
directory anymore and the right permissions are also handled as well.

Another advantage is that we now have an uniform management tool for
both automatic and manual config, so the documentation in the NixOS
manual now applies to the manual PKI config as well.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>