Set DisableKeepAlives to true for the http client

Without DisableKeepAlives being true, kube-register keeps all
connections to fleetd open. Eventually both exceed the limit of open
file descriptors.

Please see https://github.com/coreos/fleet/issues/1022 for some more
details.
Merge pull request #5 from bqbn/master

Set DisableKeepAlives to true for the http client