LU-15412 tests: Let init_clients_lists() export client vars

LU-15412 tests: Let init_clients_lists() export client vars

init_clients_lists() counts the value of client related variables
correctly. So let it define and export these variables.

This patch can fix sanity test 807 stuck issue when running on
multi-node Lustre cluster and CLIENTS is empty.

Also cleanup client count checking. Now CLIENTS is always set.

Change-Id: I9a5d4b9bde401e14e1d7f6f88b04c8d1c6aea11a
Signed-off-by: Xinliang Liu <xinliang.liu@linaro.org>
Reviewed-on: https://review.whamcloud.com/45994
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>