Add non-authenticated ping endpoint to agent

[olljanat]

On multi swarm setup we want to be able have hardware load balancer between Portainer and Agent.
That way we can have example three manager nodes behind of that LB and make sure that Portainer is always contacting to working node.

Configuration it selves works already using SSL bridging but problem is that LB is not able to see if agent is really up or not. That why it would be nice to have simple non-authenticated /status endpoint on agent which can example just say OK so LB knows that as long it gets this response Agent is good to be used.

Then we don't loose access to Swarm from Portainer even if one node goes down.

[deviantony]

@olljanat Please open your feature requests in our fronting repository for the whole Portainer ecosystem at https://github.com/portainer/portainer/issues

I think that someone already created one for this feature though: portainer/portainer#2288

IMO we can add a /ping endpoint just responding with a 200 that does not expect any authentication.

[deviantony]

@chiptus I think we can add the ping API endpoint and use it to tackle the agent api version handshake AND the problem raised in this issue.

API version is returned as a header as a part of all agent responses.