LU-13042 tests: give more time in sanity-selinux test_21b

LU-13042 tests: give more time in sanity-selinux test_21b

In sanity-selinux test_21b, set sepol refresh time to 1000 seconds
instead of 10. This gives plenty of time for file/dir access tests,
and also cache drop, to complete. Then reset send_sepol to a smaller,
already expired value, to force sepol refresh.

Test-Parameters: trivial
Test-Parameters: clientselinux mdtcount=4 testlist=sanity-selinux envdefinitions=ONLY="21b"
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I57f72faad4bd55736a3240cdefdac2e5814eba79
Reviewed-on: https://review.whamcloud.com/36905
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Reviewed-by: Alexey Lyashkov <c17817@cray.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>