add "pyenv local system" back to ycsb command
add "pyenv local system" back to ycsb command

Change-Id: I62d76b74debe8856f921084edf077e9fa5b05b5c
Reviewed-on: https://review.couchbase.org/c/perfrunner/+/170999
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Dragos Petru Taraban <343528c689fab55b50fddc59d1a7c0a2f2fdc7a7@couchbase.com>

couchbase/perfrunner,couchbase/perfrunner,couchbase/perfrunner,couchbase/perfrunner,couchbase/perfrunner,couchbase/perfrunner