Fixes StopIteration error in multi scaling group descriptor scenarios

The code was iterating over all vdus in a vnfd, instead of doing
so over the vdu refs inside the scaling-group. This lead to errors
when there was a vdu without the metric declared in the criteria.
It has been replaced to iterate over the vdus in the scaling group
descriptor, which is the correct behaviour.
This commit also fixes an error in the scale function, which was
still using the old db structure.

Signed-off-by: Benjamin Diaz <bdiaz@whitestack.com>
Change-Id: I8b8450534f739e6afecc2602086c868ffaee79bc