Limit number of deployment events fetched

[JosephSalisbury]

When we start, and have no etag cache, we can end up requesting a large number of deployment statuses.

Makes sense to limit this to the last 3 deployment events, to reduce api hits

[JosephSalisbury]

See https://github.com/giantswarm/draughtsman/blob/master/service/deployer/eventer/github/client.go#L135

If this array ends up with 20 deployments in it, we'll hit the api 20 times, even if we (as users) only really care about the last deployment event

[JosephSalisbury]

So, maybe just limiting it to the last deployment event on boot, and then how ever many we get afterwards? Uncertain.