UFW rules which allow backend apps redis access

Give access to both backend boxes. Moving forward we should think about
creating a separate backend network to make these changes easier - if we
ever spin up a new backend box it will almost certainly be forgotten to
add the firewall rule in here as well.
Mount elasticsearch data dir with lvm

Do this on all machines in the cluster rather than just 3.
It was put in to test first.
Merge pull request #516 from alphagov/elasticsearch-lvm

Mount elasticsearch data dir with lvm
Merge pull request #513 from alphagov/allow-redis-from-backend-apps

UFW rules which allow backend apps redis access