Added support for instance tag constraints in EC2 Discovery

The `aws` data bag can now have a structure at `discovery.ec2.tag` that contains EC2 instance tag
values keyed by tag name. Any tag specified will be required for an EC2 instance to
be discoverable by the plugin.

Closes #66

attributes/aws.rb

@@ -21,4 +21,9 @@
 default.elasticsearch[:cloud][:aws][:region]         = ( aws['cloud']['aws']['region']         rescue nil )
 default.elasticsearch[:cloud][:ec2][:endpoint]       = ( aws['cloud']['ec2']['endpoint']       rescue nil )
 
+discovery_tags = ( aws['discovery']['ec2']['tag'] rescue [] )
+discovery_tags.each do |tag_name, tag_value|
+  default.elasticsearch[:discovery][:ec2][:tag][tag_name] = tag_value
+end
+
 default.elasticsearch[:cloud][:node][:auto_attributes] = true

templates/default/elasticsearch.yml.erb

@@ -86,7 +86,6 @@
 <%= print_value 'discovery.zen.ping.unicast.hosts' -%>
 
 <%- if node.elasticsearch[:cloud] -%>
-# AWS Discovery
 <%= print_value 'cloud.node.auto_attributes' -%>
 <%= print_value 'cloud.aws.access_key' %>
 <%= print_value 'cloud.aws.secret_key' %>
@@ -97,6 +96,9 @@
 <%= print_value 'discovery.ec2.availability_zones' -%>
 <%= print_value 'discovery.ec2.any_group' -%>
 <%= print_value 'discovery.ec2.ping_timeout' -%>
+<%- node.elasticsearch[:discovery][:ec2][:tag].sort.each do |key, value| -%>
+discovery.ec2.tag.<%= key %>: <%= value %>
+<%- end unless node.elasticsearch[:discovery][:ec2][:tag].keys.empty? rescue false -%>
 <%- end -%>
 
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