Add new HostConfig field, Mounts.

`Mounts` allows users to specify in a much safer way the volumes they
want to use in the container.
This replaces `Binds` and `Volumes`, which both still exist, but
`Mounts` and `Binds`/`Volumes` are exclussive.
The CLI will continue to use `Binds` and `Volumes` due to concerns with
parsing the volume specs on the client side and cross-platform support
(for now).

The new API follows exactly the services mount API.

Example usage of `Mounts`:

```
$ curl -XPOST localhost:2375/containers/create -d '{
  "Image": "alpine:latest",
  "HostConfig": {
    "Mounts": [{
      "Type": "Volume",
      "Target": "/foo"
      },{
      "Type": "bind",
      "Source": "/var/run/docker.sock",
      "Target": "/var/run/docker.sock",
      },{
      "Type": "volume",
      "Name": "important_data",
      "Target": "/var/data",
      "ReadOnly": true,
      "VolumeOptions": {
	"DriverConfig": {
	  Name: "awesomeStorage",
	  Options: {"size": "10m"},
	  Labels: {"some":"label"}
	}
      }]
    }
}'
```

There are currently 2 types of mounts:

  - **bind**: Paths on the host that get mounted into the
    container. Paths must exist prior to creating the container.
  - **volume**: Volumes that persist after the
    container is removed.

Not all fields are available in each type, and validation is done to
ensure these fields aren't mixed up between types.

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

volume/validate_test_unix.go

@@ -0,0 +1,8 @@
+// +build !windows
+
+package volume
+
+var (
+	testDestinationPath = "/foo"
+	testSourcePath      = "/foo"
+)