Add module for loading specific classes
Add module for loading specific classes

This adds nova/loadables.py which contains code that is to be shared by
host and cell scheduling filtering and weighing.

Most of this code originated from nova/scheduler/filters/__init__.py
(which was copied from nova/api extension loading).

This makes it more generic so that it can be shared.  Note that this
functionality is quite different than the generic plugin manager that
exists in openstack-common.  That code could not be used here without
some significant changes.  It also seems pretty overkill for the
functionality required by scheduling filtering and weighing.

Change-Id: I1b217dc2bc2d1dc2235c8251318d06b46597e8f4

n0ano/gantt,n0ano/gantt