Add an example domain-ec2.xml file to the docs/examples/config directory

The docs/examples/config directory already contains an example standalone-ec2-ha.xml file and an example standalone-ec2-full-ha.xml file. Once an example domain-ec2.xml file has also been added to this directory, the EAP 7.1 AMIs will be able to switch over to using these EC2 example configuration files directly instead of manually creating custom example EC2 configuration files.

My current changes to get a domain-ec2.xml file added can be seen here:
https://github.com/fjuma/wildfly/commit/602e5e78c3a6887acab07a28d3439aad3f2ff062