Add support for bindmounts in ToolConfig

[prioux]

Our ToolConfig objects have an attribute where the admin can configure filesystem overlays, using a text box with e.g.

file:/some/path/thing.sqs # explicit squashfs file on remote side
userfile:12345            # ID of userfile containing the squashfs file

I'd like us to support simple bindmounts too; the ClusterTask class already has support for these when they come from local DPs, adding more would be convenient. I suggest a specification like:

bindmount:/bourreau/path/to/thingie:/containerized/path/to/thingie