Bug 22892: Remove warning from rebuild_es if --processes not passed

Bug 22892: Remove warning from rebuild_es if --processes not passed

Use of uninitialized value $processes in numeric lt (<) at
misc/search_tools/rebuild_elasticsearch.pl line 199.

We want the number of processes to be set to 1 by default, and then
assign it to $slice_count

Test plan:
Run the script with and without --processes and confirm that the warning
went away.

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>