Treat unbound port

Following a nice suggestion by S Orlando regarding the fix for
bug 1173463

Change-Id: I2fa2a2fa7852025f0c18ff27c5fffc5667ce8378
Fix usage of Query.all() and NoResultFound

SQLAlchemy Query.all() method doesn't raise NoResultFound
exception in case if no result is found but rather returns
an empty list. So this pattern which is common for our code
doesn't make sense and should be fixed:

    try:
        dns_qry = context.session.query(models_v2.DNSNameServer)
        return dns_qry.filter_by(subnet_id=subnet_id).all()
    except exc.NoResultFound:
        return []

Fixes bug 1173131.

Change-Id: I19b64eecd6edac0dba7dd0bf2ab32690c46e4f8b
Merge "Fix usage of Query.all() and NoResultFound"
Merge "Treat unbound port"