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