Shorter proxy description syntax

Merge !516 (merged) first!

This MR reworks the proxy component and proposes a shorter syntax close to what was proposed here: !516 (comment 191572)

The shorter possible being:

component: proxy

proxies:
  - name: myservice
    target: http://localhost:8000/
    openapi: http://localhost/openapi.json

which exposes endpoints defined in openapi.json as sub endpoints of /api/proxy/myservice/.

I also added loading openapi.json from file. @valentin.valls I let you add the load from config you want to do.

I dropped the support of query: to specify the params, since openapi does that well. Let me know if query: is needed, I'll restore it.

I was also wondering whether or not to remove routes: and only relies on a openapi.json file to describe what is exposed. What do you think?

attn @valentin.valls