[MERGE][#250422] implement RemoteBzrDir.get_config(), allowing push over the hpss to see a default stacking policy.

[Ian Clatworthy]

Ian Clatworthy has voted approve.
Status is now: Approved
Comment:
I'll merge.

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C48852A7B.8010602%40canonical.com%3E