Enhancing --install-latest-release

[jfgrimm]

With the EB 5.0 beta-release, we are having to be a little creative with naming to avoid that eb --install-latest-release defaults to a beta release, rather than the latest stable release.

To avoid the need for this in future, we should enhance the framework code to pick the latest stable release by default, and perhaps allow e.g.

eb --install-latest-release=beta

to allow users to explicitly pick alpha/beta/rc versions instead