Variable interpolation in env definitions

[hlubek]

Environment variables will be inherited from the process and can be set/overriden for pipelines and tasks from the definition.

Currently they are not using variables that can be passed to a job when scheduling pipelines.

Idea: Interpolate variables (using the same text/template mechanism) in environment variable values (to be defined: either only env vars from pipeline / task definition or all env vars effectively set).

We should also discuss whether this should be the default or opt-in.