Use FQDNs when referencing cluster machines.

The following points are worth recording for future reference:

* To make sure ansible reports the correct FQDN for each machine, the
  FQDN needs to be present in the /etc/hosts file. Even if the hostname
  of a machine is set to the FQDN, ansible cuts it short at the first
  period. I'm not sure I like this, but that's how it is even, after a
  fair amount of discussion on ansible's devel list and github issues;
* The rsyslog.conf files also change to look for files with the FQDN in
  them because the behaviour of the hadoop ecosystem tools is to use the
  full hostname (not truncated) output of the 'hostname' command to
  determine the name of the log files.
Use a specific version of logstash-index-cleaner to avoid repo changes breaking the play.
Turn on ansible accelerate mode with an envvar, defaulting to off.

So running script like this turns accelerate on:
  $ ACCEL=true ./site_alternative.sh