Make ________Interface.system a required field
Eliminate now-redundant validation in clean()
Fix broken tests; do miscellaneous cleanup

Conflicts:
	cyder/cydhcp/interface/static_intr/tests/aux_attrs.py
	cyder/cydhcp/interface/static_intr/tests/basic.py
	cyder/cydhcp/interface/static_intr/tests/ipv6_basic.py
	cyder/cydhcp/lib/tests/intr_from_domain.py
Merge pull request #269 from drkitty/system-not-null_v2

Simplify StaticInterface.system and DynamicInterface.system by using Django's built-in required field functionality (v3)