netcmd: Include num-members in 'samba-tool group list --verbose'

netcmd: Include num-members in 'samba-tool group list --verbose'

This adds an easy way for users to see (via samba-tool) how many members
are in various groups, without querying the members for each individual
group.

For example, you could pipe this output to grep to check for groups with
zero or one members (i.e. historic groups that may no longer make
sense).

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>