MT#10021 get_default_pingnodes: add dns parameter

dns = 'yes' returns gateway + dns
dns = 'no'  returns gateway

Change-Id: I44a74c457b8b4c03a0385fb1aae1566063d5272b
(cherry picked from commit fb66d58253b2a2f1a4cc0ca784c21b5441b9a4c2)