Implement plugins for logging drivers

Logging plugins use the same HTTP interface as other plugins for basic
command operations meanwhile actual logging operations are handled (on
Unix) via a fifo.

The plugin interface looks like so:

```go
type loggingPlugin interface {
  StartLogging(fifoPath string, loggingContext Context) error
  StopLogging(fifoPath)
```

This means a plugin must implement `LoggingDriver.StartLogging` and
`LoggingDriver.StopLogging` endpoints and be able to consume the passed
in fifo.

Logs are sent via stream encoder to the fifo encoded with protobuf.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

integration-cli/docker_cli_plugins_logdriver_test.go

@@ -0,0 +1,27 @@
+package main
+
+import (
+	"strings"
+
+	"github.com/docker/docker/integration-cli/checker"
+	"github.com/go-check/check"
+)
+
+func (s *DockerSuite) TestPluginLogDriver(c *check.C) {
+	testRequires(c, IsAmd64, DaemonIsLinux)
+
+	pluginName := "cpuguy83/docker-logdriver-test:latest"
+
+	dockerCmd(c, "plugin", "install", pluginName)
+	dockerCmd(c, "run", "--log-driver", pluginName, "--name=test", "busybox", "echo", "hello")
+	out, _ := dockerCmd(c, "logs", "test")
+	c.Assert(strings.TrimSpace(out), checker.Equals, "hello")
+
+	dockerCmd(c, "start", "-a", "test")
+	out, _ = dockerCmd(c, "logs", "test")
+	c.Assert(strings.TrimSpace(out), checker.Equals, "hello\nhello")
+
+	dockerCmd(c, "rm", "test")
+	dockerCmd(c, "plugin", "disable", pluginName)
+	dockerCmd(c, "plugin", "rm", pluginName)
+}