AwsEc2UnicastHostsProvider should use version.minimumCompatibilityVersion()

The AwsEc2UnicastHostsProvider creates DiscoveryNodes that are used as an initial seed for unicast based discovery. At the moment it uses Version.CURRENT (see [1]) for those DiscoveryNode object, which confuses the backwards compatibility layer to think this nodes are of the latest version. This causes new nodes to fail to join old nodes as the ping serialization goes wrong. Instead we should use version.minimumCompatibilityVersion(). See [2]

[1] https://github.com/elasticsearch/elasticsearch-cloud-aws/blob/es-1.x/src/main/java/org/elasticsearch/discovery/ec2/AwsEc2UnicastHostsProvider.java#L165
[2] https://github.com/elasticsearch/elasticsearch/blob/master/src/main/java/org/elasticsearch/discovery/zen/ping/unicast/UnicastZenPing.java#L130

Ps. this was reported on the mailing list. See: https://groups.google.com/forum/#!msg/elasticsearch/8pUwFld88tI/7jRuG6hqtbAJ

Closes #143.