Use http client created in Swarm

[nishanttotla]

The changes made in #2206 to address #1879 were lost recently. This PR re-introduces those changes. There was a similar effort in #2555 but that has stalled.

This also prevents creating unnecessary extra http connections.

Signed-off-by: Nishant Totla nishanttotla@gmail.com

[nishanttotla]

Ping @allencloud

[allencloud]

I think it seems that we vendored a new version of package samalba/dockerclient. Do we ignore that kind of information now? @nishanttotla

[nishanttotla]

@allencloud see #1879. samalba/dockerclient should be removed soon, and this is a step in that direction. The reason we're changing the vendored directory temporarily is because it makes no sense to submit a PR to samalba/dockerclient at this stage, and so we should just consider this a temporary measure.

[xihan88]

@nishanttotla A small issue related: In func HTTPClientAndScheme (cluster/engine.go), there is no need for type check since httpClient and url.Scheme are both valid in engine. If do need a check would it be SwarmClient instead of DockerClient I guess?

[nishanttotla]

@xihan88 the check is needed to make sure it isn't a NopClient (which is the case when connection has been lost)

[nishanttotla]

@allencloud updated request default timeout to 30 seconds.

[nishanttotla]

cc @wsong. This PR is the last step before we can get rid of samalba/dockerclient. While I'm confident no old API requests due to that are being sent anymore, it would be great to entirely get rid of this dependency.