Code review comment for lp:~johnsca/charm-helpers/services-callback-fu

*** Submitted:

Callbacks instead of ServiceManagers

Refactored charmhelpers.core.services to use callbacks instead of
service types / managers, to make it easier for charms to specify
custom behavior.

R=benjamin.saller
CC=
https://codereview.appspot.com/98490043

https://codereview.appspot.com/98490043/