Add custom userdata

[nicklester]

Can this be extended so the template user can add more user data to allow the bastion host to, for example, perform initialisation/install custom software?
Happy to work on a PR if we think this is a wise idea?

[michaelwittig]

Either UserData or we could also enable ssm. This would allow you to work with SSM State Manager to install additional stuff. Both ways are available in other modules, e.g. https://github.com/cfn-modules/asg-singleton-amazon-linux2/blob/master/module.yml