LU-10907 tests: improve nodemap_test_setup()

LU-10907 tests: improve nodemap_test_setup()

Do wait_nm_sync() after modifying default.admin_nodemap, otherwise
it might cause the latter sync to fail.

Also, this patch checks the number of clients for sanity-sec.sh
test_23a, 2 clients at least for nodemap c0 and c1.

Test-Parameters: trivial testlist=sanity-sec

Change-Id: I7c7d5929b2f2b5aaaa0e7e884fa296fdf3cc1b57
Signed-off-by: Emoly Liu <emoly.liu@intel.com>
Reviewed-on: https://review.whamcloud.com/32018
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Saurabh Tandan <saurabh.tandan@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>