LU-1189 tests: run save_lustre_params() on facets

LU-1189 tests: run save_lustre_params() on facets

This patch fixes save_lustre_params() and restore_lustre_params()
to get and set Lustre parameters through do_facet() instead of
do_node(). This will fix the issue that restore_lustre_params() still
tries to restore the saved params on the original node which will
likely become inactive under failover test environment.

Test-Parameters: envdefinitions=SLOW=yes,ENABLE_QUOTA=yes \
clientcount=4 osscount=2 mdscount=2 austeroptions=-R \
failover=true useiscsi=true \
testlist=replay-vbr,replay-dual

Signed-off-by: Jian Yu <jian.yu@intel.com>
Change-Id: If73adb84c4f1b2f6d6667a3865f2cc0221f0e687
Reviewed-on: http://review.whamcloud.com/5627
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Minh Diep <minh.diep@intel.com>
Reviewed-by: Li Wei <wei.g.li@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>