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StorageOS configurable device directory and mount options

**What this PR does / why we need it**:
This allows StorageOS volumes to be mounted when the kubelet is running in a container and we are unable to use the default device location (/var/lib/storageos/volumes).  With this PR, the node's device location is requested via the StorageOS api, falling back to the current behaviour if not configured.  The node's device location can be supplied as an environment variable (DEVICE_DIR) to the StorageOS container.  This is backwards-compatible and no changes are needed to existing deployments.

The PR also allows Mount options to be set for StorageOS volumes in the same way they're enabled for other volume plugins.

The StorageOS API dependency was updated to the latest version, but no functionality changes besides adding the DeviceDir property to the Controller object.

There is also a small refactor of the loopback device handling code in storageos_utils.go to capture stderr output.

**Release note**:
```release-note
StorageOS volume plugin updated to support mount options and environments where the kubelet runs in a container and the device location should be specified.
```

Not sure why godep changed the comments of unrelated packages in Godeps.json...

/sig storage