Vista must work with the distributed Fn

[denismakogon]

So, at this moment, Flow mode is the only way to run vista demo.
At this moment in order to run vista on a remote Fn a lot of work needs to be done:

• build/test images
• push images to the registry
• setup an application with some functions

For now Vista in Flow mode works only with all-in-one Fn (basically what you get from fn start).
However, Vista suppose to work fine with distributed Fn (api, runner lb, runners).
So, how things suppose to be different:

• flow needs to talk to LB runner (an address that is mentioned in invoke annotation)
• new env var needs to be introduced FN_INVOKE_PATH that by default should point to Fn API endpoint.

Hopefully, this is the amount of work that needs to be done.