cli: new "node prune" command
Used to remove shared parts that are no longer referenced.
With shared part installed for example in /opt/slapgrd/shared/name/HASH
the heuristic is to assume that if HASH is present in a software's
.installed.cfg, then this software is using this shared part.
Similar, if 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
contain a 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.