Issue #35 - dsconf automember support
Bug Description: Add support for managing automember to dsconf
Fix Description: Initial patch which adds AutoMembershipPlugin, AutoMembershipDefinition
and AutoMembershipDefinitions classes to plugins.py and adds tests for checking valid scope,
valid filter and if user is correctly added to the group.
https://pagure.io/lib389/issue/35
Author: Alisha Aneja
Review by: ???