Turning this into a parameter may allow for undesired levels of version flexibility.  A user could pass in a version of Tomcat 7 or 8, for example, with which the app may not function properly.  I think it is still good to abstract this away from the user, since they really shouldn't need to think about Tomcat versions when launching an EMR cluster with Genie, but it is also not ideal to have such a brittle configuration.  

The best option may be to bypass the mirror altogether and go straight to the archive distribution, which doesn't inconveniently disappear with each new release.  That way, the TOMCAT_VERSION could be moved forward when necessary, but the bootstrap process won't break if it ever falls behind.

I'll commit that change for review.