samba-tool: Update 'samba-tool gpo list <>' description

samba-tool: Update 'samba-tool gpo list <>' description

We have a command to get gpo listing from Active Directory.
samba-tool gpo list <username>

This command can list GPOs for both username and machinename,
But command help only shows 'username'.

This PR
- Updates the option presented in help.
- Updates name of variable used to retrieve GPO so that it's
 not misleading if someone reads code later on

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14016

Signed-off-by: Amit Kumar <amitkuma@redhat.com>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andreas Schneider <asn@samba.org>
Autobuild-User(master): Andreas Schneider <asn@cryptomilk.org>
Autobuild-Date(master): Thu Oct 17 09:21:20 UTC 2019 on sn-devel-184