LU-2309 config: ignore unknown configuration param

LU-2309 config: ignore unknown configuration param

Client or server should not fail to mount if it hits
a configuration parameter that it doesn't understand.
This patch fixes class_process_config() to meet
the above requirement.

The patch also improves conf-sanity test 42 to verify
that invalid sys config param should not prevent client
or server from mounting.

Signed-off-by: Jian Yu <jian.yu@intel.com>
Change-Id: I33a6eb3c2beb3dd7425a097e8e7b5a2766852a96
Reviewed-on: http://review.whamcloud.com/5037
Tested-by: Hudson
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>