Change node parameter oob_program to VTYPE_STRING

Since this is an inheritable parameter, having it as a MABYE_STRING
creates only problems (per our derivation rules). We change it to
STRING, with the default "", meaning no program. Note that most of the
code already accepts this as valid for "no program", and some comments
even say that this is the expected value.

We have some other parameters like this, I'll have to investigate
whether they need to be changed too. But right now I need this for the
hconfd changes (it's a prerequisite for them, I forgot to send it in
that patch series).

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