Make dashboard secret generation generic

Currently, whenever we add a secret to an app (think admin password, database password, sessions secret, hash salt) -- or add an app, for that matter -- we need to change the dashboard to create that secret on the cluster when the app is installed. That is a pain for development and makes it harder to extend or customize.

The code snippets for generating the secrets are currently completely generic, so the only remaining piece to fix this would be to have some way for the apps to specify which variables need to be populated. The dashboard would then read that specification on app install and generate the required secrets.

Perhaps the list of variable names can be part of the stackspin-apps configmap.