[17.12] Support a proxy in splunk log driver

[thaJeztah]

backport of moby/moby#36220 for 17.12

git checkout -b 17.12-backport-support-proxy-in-splunk-driver upstream/17.12
git cherry-pick -s -S -x -Xsubtree=components/engine 3c4537d5b33d951237ea5e4cc123953eda7a37e7
git cherry-pick -s -S -x -Xsubtree=components/engine 7d296522f68a80d540b20753ea0648171ee12825

no conflicts

[thaJeztah]

Whoops looks like this needs a revendor as well;

00:40:22 # github.com/docker/docker/daemon/logger/splunk
00:40:22 package github.com/docker/docker/daemon/logger/splunk (test)
00:40:22 	imports github.com/gotestyourself/gotestyourself/env
00:40:22 	imports github.com/gotestyourself/gotestyourself/assert: cannot find package "github.com/gotestyourself/gotestyourself/assert" in any of:
00:40:22 	/go/src/github.com/docker/docker/vendor/github.com/gotestyourself/gotestyourself/assert (vendor tree)
00:40:22 	/usr/local/go/src/github.com/gotestyourself/gotestyourself/assert (from $GOROOT)
00:40:22 	/go/src/github.com/gotestyourself/gotestyourself/assert (from $GOPATH)

[thaJeztah]

Added moby/moby@4ac4b69 (moby/moby#36034) for updated gotestyourself

[dnephin]

LGTM

[thaJeztah]

Test failure due to the hosted notary service having an outage this weekend;

Error:      	"unable to reach trust server at this time: 503." does not contain "you are not authorized to perform this operation: server