Remove set mountpoint dialog in Horizon

As per [ mountpoint doesn't work when a volume is attached to an instance ](https://bugs.launchpad.net/nova/+bug/1004328) and our users' experiences one can not set a mount point as we currently configure our cluster. To avoid confusing users, we should disable the dialog box in Horizon.

Should one want a deterministic handle, they can use volume labels when making a file system on the device or on Ubuntu, the `/dev/disk/by-uuid/<volume-uuid>` path should be populated.