tools: Add required VM networks config value

The OnlyRequiredNetworksMandatoryForVdsSelection value has to be
user-configurable in order to be able to run external networks which are
dynamically provisioned.

Change-Id: I257aaef3c6aacc65fe0b217f6ef6c2d866f095d9
Signed-off-by: Mike Kolesnik <mkolesni@redhat.com>