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.

/reviewed-on nexedi/slapos.core!139