create a subnet arg validator

Signed-off-by: Alfredo Deza <alfredo.deza@inktank.com>
allow passing public/private networks to new

Signed-off-by: Alfredo Deza <alfredo.deza@inktank.com>
add some tests for the dumb subnet validator

Signed-off-by: Alfredo Deza <alfredo.deza@inktank.com>