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.