Changes to support solr-bench external mode

[patsonluk]

1. Added "external" mode from #33 , with some additional enhancements:
   1. Added config external-solr-config.zk-chroot and build zk client with such chroot
   2. Allow configuration of bind port and admin port for GenericZookeeper for external mode, those 2 ports could be something other than the defaults (2181, 8080)
   3. Added support for reading zk for live query nodes as well. Use such list of query node for query benchmarking as a preference
   4. Do not attempt to upload configset if it's on "external" mode,
2. Added a flag in restart task which restarts all nodes (right now all concurrently). If there're more than one node restarted, the node-startup, node-shutdown and heap-mb will be the average, there will also be node-startup-by-node, node-shutdown-by-node and heap-mb-by-node which are maps with key as the node name with individual value per entry. There should be no change to the result for restart test on single node
3. Added "init-timestamp", "start-timestamp" and "end-timestamp" to the result for task execution. These values might be handy for adding annotation in the future
4. Add option to run stress.sh with a pre-built solr-bench binary. This can speed things up for docker environment, as attempt was made to checkout and build the solr-bench as a part of docker image build process but it's extremely slow (at least on OSX). So ended up having the Makefile to build the solr-bench jar locally first, then add to the docker image.

[patsonluk]

@chatman Can u please review these changes? I have tested on both "external" mode and "local" mode.

Tagging @hiteshk25 to keep him in the loop :)

[noblepaul]

looks good

+1

[chatman]

LGTM! Please merge, thanks!