Add D-Bus permissions for 'dhcp' user in flimflam.

BUG=chromium-os:28336
TEST=Build and boot an image, network still works.

Change-Id: I9a5e947dd157d701c50499ca9ca6f374f643817e
Reviewed-on: https://gerrit.chromium.org/gerrit/20633
Tested-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
Reviewed-by: Paul Stewart <pstew@chromium.org>
Commit-Ready: Jorge Lucangeli Obes <jorgelo@chromium.org>