Break out common functionality for filters and weights

In the context of openstack.common.scheduler:

base_filter.BaseFilterManager and base_weight.BaseWeightManager had
duplicated code.

This patch breaks that code out into a shared base class:
base_handler.BaseHandler

Change-Id: Ie5232c6dd9876a75170beda71adde90fc6bb4153
Merge "Break out common functionality for filters and weights"