netcmd: add optparse validators and Range validator
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>