CA-137722: Determining gateway and dns appropriately.

If bring_pif_up is called for management interface then passing
that pif to determine_gateway_and_dns_ifs function else passing None.

Signed-off-by: Ravi Pandey <ravi.pandey@citrix.com>
Merge pull request #1793 from ravippandey/CA-137722

 CA-137722: Determining gateway and dns appropriately.