LU-7004 quota: make lctl set_param -P functional for quota

LU-7004 quota: make lctl set_param -P functional for quota

Currently setting up quota permanently can only be done with a
command like lctl conf_param $FSNAME.quota.ost=ug. To see if those
settings take hold we examine the 'enabled' proc file located in
the quota_slave directory in the proc tree. To make this workable
with lctl set_param -P we can make the 'enabled' proc file
writable and lustre can treat the config log change from set_param
-P for quota like any other tunable to be set permanetly.

Change-Id: I6a4c1fdc9d16658930f48d21e4f79e6f36047511
Signed-off-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-on: https://review.whamcloud.com/31081
Reviewed-by: Fan Yong <fan.yong@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Wang Shilong <wshilong@ddn.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>