Remove "reuse_existing" from setup method in dhcp.py

This flag is never used, it's always set to True by
DhcpLocalProcess.enable()

Change-Id: Ic30e0f2c97679d5919cc4e4afeb38666a6d41392
Closes-bug: #1305083
Merge "Remove "reuse_existing" from setup method in dhcp.py"