Update code to properly use dict returned from get_gateway.

Calling get_gateway was a late addition to my code to add a default
route in the DHCP namespace.  The code did not properly handle the
dict returned from that method.  This changes it to properly extract
the 'gateway' attribute from the dict.

Change-Id: I9823a31feee7ab333c277fc6e78f366408393155
Fixes: Bug #1195543
Merge "Update code to properly use dict returned from get_gateway."