cli: new "node prune" command
Used to remove shared parts that are no longer referenced.
With shared part installed for example in
the heuristic is to assume that if
HASH is present in a software's
.installed.cfg, then this software is using this shared part.
HASH is present in
/opt/slapgrid/shared/another/ANOTHER_HASH/.slapos*signature then share
part another for hash
ANOTHER_HASH is using the shared part.
This also have some support to recursively look in instances and if they
slapos.cfg file, look for software and shared parts to see if
they use the shared part.
In other cases, this shared part can be deleted.