network: fix unrecognized engine values for STP

The engine sends STP settings in a 'yes'/'no' range of values, which
is disjoint from what I set as acceptable in
I6a1a37485ae40b8c9f2037c671daa3e4c6ac3f90

This patch fixes the stp booleanization so that it accepts what the
engine has always sent.

Change-Id: I5bacd30a263a3a2dcac8eb8a75ed6f68cab378be
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/27474
Reviewed-by: Dan Kenigsberg <danken@redhat.com>