4138: Revamp Node Manager driver proxying in BaseComputeNodeDriver.

4138: Revamp Node Manager driver proxying in BaseComputeNodeDriver.

Accessing attributes through a super() proxy does not invoke
__getattr__ on base classes, so the old implementation made it
impossible for subclasses to be agnostic about whether a method was
implemented in BaseComputeNodeDriver or the real libcloud driver.
This version makes that possible.  It's also a little nicer because
now the class will report these method names to dir(), hasattr(), etc.