0.9.208: Relax volume validation requirements (#1222)

* Relax volume validation requirements

This commit adds support for mounting named volumes by relaxing the
validation performed by the `JobValidator`. This allows jobs to use
options for volumes managed by a particular driver, such as:

```
docker volume create --driver local \
   --opt type=btrfs --opt device=/dev/sda2 \
   myvolume
```

The above volume can then be mounted into a container using the
following helios job configuration (full configuration omitted):

```
{
  ...
  "volumes": {
    "/mnt/myvol": "myvolume"
  }
}
```

* Address review comments