Allow for switching to different AppDynamics account by setting the "appdynamics_account_name" Taupage property only

[regispl]

If I understand correctly, at the moment AppD defaults to "production" account and Taupage is providing required account_name, access key and globalname as defaults. However, to switch to testing account I need to explicitly overwrite all these fields in my Senza config:

  appdynamics_account_name: <NAME>
  appdynamics_account_access_key: <KEY>
  appdynamics_account_globalname: <GLOBAL>

It's a bit annoying because it means that I have to either:

1. Make them all parametrised in Senza and provide them every time, with different values for deployments to "prod" and "testing"
2. Have two Senza definition YAMLs (one without these fields, that uses "defaults" and the other one that overwrites them)
   Second one is not an option, first one is quite annoying, but we can live with it.

However, I was wondering if it would be possible to make Taupage a bit "smarter" and figure out the correct credentials based on appdynamics_account_name only? If it can provide defaults now, could if provide different defaults depending on account name? This way we could have a single param (which is human readable and not a secret) that's specifying the AppD account and we wouldn't have to provide the other two (long, non-human-readable strings that need to be stored in encrypted form somewhere).