E2EConfig: Use MustGetVariable() instead of GetVariableOrEmpty()

[guettli]

/kind feature

For convenience, we use GetVariableOrEmpty() in the e2eConfig.

We should have a look which vars via actually use.

We could replace all GetVariableOrEmpty calls with MustGetVariable and look where it fails.

This would reveal a lot of dead code (like CCM with Network support).