LU-1187 tests: Add DNE tests to conf sanity.

LU-1187 tests: Add DNE tests to conf sanity.

1. 32c: upgrade 2.1/1.8 to DNE.
2. 70/71: start targets in mixed order for DNE.
3. Add start_mds to start all of MDTs during the test.
4. Fix writeconf to erase the config for all targets.

Change-Id: I62e9173d53b96082059b1faf7e0455d0c07eb967
Signed-off-by: Wang Di <di.wang@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/4367
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>