This commit ensures that when a plugin instance is closed the goroutines
responsible for streaming stdout and stderr of the called binary will
also exit, preventing a goroutines leak.
Before this commit these goroutines could stay blocked forever if
Close() was called while the binary still had some pending output.
I found this bug after debugging a real world goroutine leak, the
goroutines dump would show thousands of goroutines at:
```
github.com/tsuru/tsuru/vendor/github.com/docker/machine/libmachine/drivers/plugin/localbinary.stream(0xc023013d80, 0xc0000aad20)
/home/travis/gopath/src/github.com/tsuru/tsuru/vendor/github.com/docker/machine/libmachine/drivers/plugin/localbinary/plugin.go:177 +0x7c
created by github.com/tsuru/tsuru/vendor/github.com/docker/machine/libmachine/drivers/plugin/localbinary.(*Plugin).AttachStream
/home/travis/gopath/src/github.com/tsuru/tsuru/vendor/github.com/docker/machine/libmachine/drivers/plugin/localbinary/plugin.go:183 +0x67
```
Signed-off-by: Cezar Sa Espinola <cezarsa@gmail.com>
55bfb59b67