LU-427 test: Test failure on test suite lfsck

LU-427 test: Test failure on test suite lfsck

- Reset $MDSDB & $OSTDB in generate_db(). Else they will
  stale if user redefine $SHARED_DIRECTORY.
- Add a function check_shared_dir() to ensure
  $SHARED_DIRECTORY is shared among tests nodes.
- Fix check_logdir() and check_write_access() to avoid using
  node.$(hostname).yml files which should not be deleted.

Signed-off-by: Yang Sheng <ys@whamcloud.com>
Signed-off-by: Yu Jian <yujian@whamcloud.com>
Change-Id: Ie7d1d08c0d2c701fa9fb74ef8b252fa8b31bf111
Reviewed-on: http://review.whamcloud.com/2498
Tested-by: Hudson
Reviewed-by: Wei Liu <sarah@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Johann Lombardi <johann@whamcloud.com>