I've been having similar issues with ec2 clustering discovery. Using the following guide and modifying according to the changes from 1.4.4 to 1.5.2 I've got all the different boxes up and running on AWS, but apparently the two elasticsearch nodes aren't joining into a cluster.
Does the IAM user have access to describe instances?
Does the security group allow traffic on 9300 to instances within the security group?
Did you upgrade the aws plugin when you upgraded ElasticSearch?
Do both instances have "ELK - Elasticsearch" as their name?
The config I posted runs fine on es 1.5.2 within an aws vpc
I got my issue resolved. I was using an IAM key pair, but needed my AWS root keys. Once that change was made my devices clustered correctly with the yml file posted above.
To answer the questions:
IAM user was the issue (see above)
Traffic is allowed on 9200, 9300, and 5200
Clean install of all current versions pulled from the elastic.co site
Yes, both instances have "ELK - Elasticsearch" as their names
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