shill: DHCPConfig: Remove use of minimal config

This reverts commit 397f1892a0f1042fba4b3d8e8fa5d57569deaab1,
but retains the DHCPArgs matcher in dhcp_config_unittest.cc.

BUG=chromium:360452
TEST=Unit tests

Change-Id: I18ab78a8e9e66a102a910a7116e80598a2892944
Reviewed-on: https://chromium-review.googlesource.com/205589
Reviewed-by: mukesh agrawal <quiche@chromium.org>
Tested-by: Paul Stewart <pstew@chromium.org>
Commit-Queue: Paul Stewart <pstew@chromium.org>