Added WaitForElasticSearchClusterGreen task
Added CullElasticSearchInstancesWithTerminationTag

Unlike the ASG culling which terminates all instances at the same
time, this waits for the cluster to go green before culling each
node.  This is therefore much slower, but hopefully safer.
Merge branch 'master' into pw-elasticsearch
Make sure all the nodes are in the cluster
Merge branch 'master' into pw-elasticsearch
Break out an ElasticSearchNode class

Tasks calling other tasks isn't an established pattern and this
didn't seem to warrant making it one
Merge pull request #61 from guardian/pw-elasticsearch

Elasticsearch