LU-3285 tests: disable kernel flusher in discard tests

LU-3285 tests: disable kernel flusher in discard tests

Test 101 checks that client discards data instead of flush,
but it is not reliable because write RPC may happens before
unlink command is executed due to kernel flusher.
Set /proc/sys/vm/dirty_writeback_centisecs to 0 for the test
time and return it back to the default value later.

Signed-off-by: Mikhail Pershin <mike.pershin@intel.com>
Change-Id: I2c4da039a456519e25c6d9ffcc015c5baa479740
Reviewed-on: https://review.whamcloud.com/30016
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>