CreateInstance: allow no ip check with start mode

Since gnt-instance start doesn't do any checks on the IP, it doesn't
make much sense to do so in instance create (with start) if the user
expressly passes in ‘--no-ip-check’. Removing this requirement eases the
no-name-check mode (otherwise one would have to create --no-start and
then only start).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>