Set concurrency 2 for CUSTOM run

Change-Id: I0bf76d42b93f18f49af340e404d025274cc8bd2f
diff --git a/tcp_tests/managers/openstack_manager.py b/tcp_tests/managers/openstack_manager.py
index 1699be5..afbd331 100644
--- a/tcp_tests/managers/openstack_manager.py
+++ b/tcp_tests/managers/openstack_manager.py
@@ -120,9 +120,8 @@
             cmd = ("docker run --rm --net=host  "
                    "-e TEMPEST_CONF={0} "
                    "-e SKIP_LIST=mcp_skip.list "
-                   "-e CONCURRENCY=2 "
                    "-e SOURCE_FILE=keystonercv3  "
-                   "-e CUSTOM='--pattern {1}' "
+                   "-e CUSTOM='--pattern {1} --concurrency 2' "
                    "-v /root/:/home/rally  "
                    "-v /var/log/:/home/rally/rally_reports/ "
                    "-v /etc/ssl/certs/:/etc/ssl/certs/ {2} >> image.output"