EX-2745 tests: limit debug log entries for hot-pools.sh

EX-2745 tests: limit debug log entries for hot-pools.sh

This patch adds --since "$duration seconds ago" option
to journalctl to gather lamigo and lpurge service debug
logs for the exact runs.

Test-Parameters: trivial testlist=hot-pools,hot-pools
Test-Parameters: trivial testgroup=review-dne-part-2

Change-Id: Idbb81bc4dc7fc669074c1e8f8c156627abe6c610
Signed-off-by: Jian Yu <yujian@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/43208
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: John L. Hammond <jhammond@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Li Xi <lixi@ddn.com>