nomad/client take very long and exceed 15m sometimes:

In https://travis-ci.org/hashicorp/nomad/jobs/452990197 :

```
panic: test timed out after 15m0s

goroutine 4739 [running]:
testing.(*M).startAlarm.func1()
	/home/travis/.gimme/versions/go1.11.2.linux.amd64/src/testing/testing.go:1296 +0xfd
....
goroutine 4665 [select]:
github.com/hashicorp/nomad/vendor/google.golang.org/grpc.newClientStream.func5(0xc0003dd500, 0xc000420120, 0x2b3f86295588, 0xc000496810)
	/home/travis/gopath/src/github.com/hashicorp/nomad/vendor/google.golang.org/grpc/stream.go:287 +0xd7
created by github.com/hashicorp/nomad/vendor/google.golang.org/grpc.newClientStream
	/home/travis/gopath/src/github.com/hashicorp/nomad/vendor/google.golang.org/grpc/stream.go:286 +0x842
FAIL	github.com/hashicorp/nomad/client/driver	900.036s
```