Add domain-specific cache tags to config overrides

Any idea how we can test this?

To test it, just make something renderable and cachable that depends on a config. Render it so it's cached. Change the config for a specific domain. The renderable should be invalidated and therefor be rendered again, now with the domain specific changes. If not, the cache has not been invalidated correctly.

This is a must have. Works fine. Should be merged!

Not sure it works in fact, will need to do more testing.

Seems to be working fine finally. Setting it back to RTBC.