idviews: Set proper objectclass for the ID override objects

Part of: https://fedorahosted.org/freeipa/ticket/3979
hostgroup: Add helper that returns all members of a hostgroup

Part of: https://fedorahosted.org/freeipa/ticket/3979
hostgroup: Remove redundant and star imports

Part of: https://fedorahosted.org/freeipa/ticket/3979
hostgroup: Selected PEP8 fixes for the hostgroup plugin

Part of: https://fedorahosted.org/freeipa/ticket/3979
idviews: Add ipa idview-apply and idview-unapply commands

Part of: https://fedorahosted.org/freeipa/ticket/3979
idviews: Extend idview-show command to display assigned idoverrides and hosts

Part of: https://fedorahosted.org/freeipa/ticket/3979
ipatests: Add xmlrpc tests for idviews plugin