Schema for runtime variables

[maany]

Is your feature request related to a problem? Please describe.
The runtime variables are bound to a repository and at present the config-schema files in repositories do not contain a schema for them. Where to place the schema for the runtime vars is a strategic decision we must make.

[maany]

Also, more importantly, make sure that runtime variables are provided a value, else fail. They are gonna be of type {use_default: false, required: true}