.ebextensions/tomcat-settings.config
option_settings: aws:elasticbeanstalk:environment:proxy: GzipCompression: 'true' ProxyServer: nginx aws:elasticbeanstalk:environment:proxy:staticfiles: /static: static
aws:elasticbeanstalk:environment:proxyのProxyServerに「nginx」を指定します。指定してない場合はApache2.2がプロキシサーバーとなります。
参考
http://docs.aws.amazon.com/ja_jp/elasticbeanstalk/latest/dg/command-options-specific.html#command-options-java
http://docs.aws.amazon.com/ja_jp/elasticbeanstalk/latest/dg/java-tomcat-platform.html