It makes more sense for these to exist in the top package, because they are used by SambaOption.

validators.py can still exist in netcmd, just not the base class and exception.

Signed-off-by: Rob van der Linde <rob@catalyst.net.nz>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>

Add the ability to the add validators to optparse Option fields.

The Option class was already subclassed in `netcmd/__init__.py` so
adding some functionality to this was relatively easy.

Added the ability to add Validator classes to a field so that this can
be used for anything else in the future, but for now there is a Range
validator required by upcoming auto silo commands.

Signed-off-by: Rob van der Linde <rob@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Joseph Sutton <josephsutton@catalyst.net.nz>