Definition is not parametrised enough to be usable "out-of-the-box"

[regispl]

We've been using a modified (not forked, just "snapshotted" and modified) version of your appliance for a while, but recently we noticed that it's getting more and more painful for us to update it, so we decided to step back, fork your appliance and make only the minimum of required changes to make it useful to us (so we don't have to maintain our own, completely different Senza definition).

Having these changes done now, it looks like they're quite "generic", so I'm thinking if that's something that could be useful for other people and something that's worth being added to your repo? I'm not opening a PR yet to get your input on the changes here first.

The changes are:

• adding InstanceType parameter to make the EC2 instance type configurable via Senza argument - for obvious reasons it's necessery to parametrise it to make the appliance "useful" to anyone out-of-the-box, without local modifications
• adding AppDynamicsApplication parameter to make Taupage's appdynamics_application configurable - I'm not sure what was the reason for using what's already there as default, but it seems like this default may not work for everyone as the name of the application might be constrained and developers just need to use whatever they were told to use, so it looks like parametrising this setting is a good idea too

Saying this I must make clear that we're not in production yet, so these changes may not be all the changes we'll have to make to make Exhibitor work for us (e.g. I EBS size may need to be configurable too?), but it was the bare minimum we had to do to even get it deployed and working.

What do you think? Would these changes make sense to you?