Make zookeeper version adjustable (ZK_VERSION env)

[adsr]

This patch parameterizes the ZooKeeper version. Confirmed working with ZK_VERSION=3.5.4-beta. There is a comment about updating the Dockerfile but I'm not so familiar with that.

Not included in this patch but related:

diff --git a/config/zkcfg/zoo.cfg b/config/zkcfg/zoo.cfg
index 8b1b00994..b8f811706 100644
--- a/config/zkcfg/zoo.cfg
+++ b/config/zkcfg/zoo.cfg
@@ -1,9 +1,10 @@
 tickTime=2000
 dataDir={{.DataDir}}
 clientPort={{.ClientPort}}
 initLimit=5
 syncLimit=2
 maxClientCnxns=0
 {{range .Servers}}
 server.{{.ServerId}}={{.Hostname}}:{{.LeaderPort}}:{{.ElectionPort}}
 {{end}}
+4lw.commands.whitelist=conf,cons,crst,dump,envi,ruok,srst,srvr,stat,wchs,mntr

The default config for 4lw.commands.whitelist is different in 3.5+. The above diff explicitly sets it to the default of 3.4. Without this, the ruok healthcheck fails. Alternatives: (a) parameterize (e.g., {{.FourLetterWords}}), (b) ability to add arbitrary config, (c) something else?

[rafael]

We should also add installation of ant to bootstrap_vm.sh in vagrant scripts. So Vagrant setup does not break after merging this.