updating elasticsearch-head org
bumping ES version
bug fix for ES plugins
version bump for mysql and zabbix to satisfy CI
mysql passwords now fixed, and bug fix for mysql::server recipe

I'm not sure if this is the result of the recent Chef attribute
overhaul, but the mysql server_root_password seems to be being
overridden with every Chef run. This has caused all kinds of grief, so
I'm going to go ahead and require that these attributes be overridden.