Fix setting ipolicy on node groups

On node groups we don't have the std field. However, the InstancePolicy
object always verifies that the std value is within a given range. As we
fill it up with defaults if not set (as it happens to be on node groups)
and the min value is higher than the default std value (taken from
constants.py) we fail.

We overcome this situation by simply let the function know if we want to
verify the std value at all. If we don't want to verify std, we just set
it to a compliant value (min_v) and continue.

We also slightly adapt the error message provided, as we don't have std
values on groups.

Signed-off-by: René Nussbaumer <rn@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>