Add a config decorator

This decorator converts arguments to config options than can be
specified in the config file.  The arguments are passed in as defaults
so they can be overridden by calling fuctions.

@configurable('connection')
def connect(uri):
    pass

[cfg:module.connection]
uri = mysql://user:password@localhost/db

Change-Id: I67af572a1b785695ca25209f46bd76df4f14b35b
Option to drop to pdb shell on error

Change-Id: I7a4592cf9f2e0842fb6f307d5518be0f0d9324af
Merge changes I7a4592cf,I67af572a

* changes:
  Option to drop to pdb shell on error
  Add a config decorator