shill: Set interface MAC address for RTNL handler

This is equivalent to ip link dev $INTERFACE address $MAC_ADDRESS

BUG=b:112327498
TEST=tested on biggie and gale.
Create a station interface, set MAC address and observe by ifconfig and
iw dev output.

Change-Id: I7cd83814a5d721cb71c4030a4a6669a046d3d958
Reviewed-on: https://chromium-review.googlesource.com/1166222
Commit-Ready: Julan Hsu <julanhsu@google.com>
Tested-by: Julan Hsu <julanhsu@google.com>
Tested-by: Julan Hsu <julanhsu@chromium.org>
Reviewed-by: Brian Norris <briannorris@chromium.org>