'Look for legacy-style (non-subnetwork style) network.
return: [list of AuditIssues]'

def _legacy_exists(self, network, error_cat='NET'):

    errors = []
    subnetworks = network.get('Subnetworks', None)
    auto_create_subnetworks = network.get('AutoCreateSubnetworks', None)
    if ((subnetworks is None) and (auto_create_subnetworks is None)):
        ae = make_audit_issue(error_cat, 'EXISTS', 'LEGACY')
        errors.append(ae)
    return errors
