Show raw config values via REST API

Makes it almost identical to how sensors does.

Also removes the `censoredValue` renderer hint from the `BrooklynNode.MANAGEMENT_PASSWORD` as entitled users need a good way to access this via the GUI; @grkvlt any reason not to do this?  (And can you spot the connection?  We've gone from one censored value to another sensor value mechanism... ha ha)

@sjcorbett @neykov can you review?  should be simple.