Extend staticHeaders functionality about gateleen

[lbovet]

I like the backward compatible syntax.
Actually, I would use an enum instead of a boolean:

• replace: write only if already exists
• complete: write only if not yet present
• default: write it in any case
  Please also take care that staticHeaders stuff is also present in hooks.

[lbovet]

Actually, there is a problem with the union-like typing (either string or object). It prevents to generate code mapping for typed language (e.g. jackson in Java).
It would be better to have another object instead of "staticHeaders".
We could use "headers", since there is a possibility to have more features in the future that will be maybe more dynamic.

[oli-h]

We have an additional enhancement proposal concerning "staticHeaders": use variables to enable dynamic header values. Example:

"headers": {
  "x-queue": { "value": "my-queue-for-{x-deviceid}", condition: "..." }
}

The syntax {x-deviceid} will be replaced by dynamically from the (assume to exist) value of header x-deviceid.
It would be very fancy if we could also reference query parameters and URL parts as dynamic values.

And not to forget: We have those "staticHeaders" (or new just "headers") no only in static routing (https://github.com/swisspush/gateleen/blob/develop/gateleen-routing/README_routing.md) but also with Hooks (https://github.com/swisspush/gateleen/blob/develop/gateleen-hook/README_hook.md).
We should do it in both modules the same way

Do we need to pay attention for cyclic references?

"aaa": {value: "{bbb}"},
"bbb": {value: "{aaa}"}