Upstart code cleanup, init provider improvement

This commit adds an is_init? function to the init provider, to prevent the init
provider from handling upstart jobs redundantly (which happens with services
such as network-interface and network-interface-security). It also adds tests
for the exlusion of instances in the upstart provider and exclusion of upstart
services from the init instances. It also cleans up some upstart provider code
(self.instances, self.search), eliminating redundant code and refactoring some
methods (upstart_status, status, normal_status).
This also removes the custom status command from upstart, which almost
certainly wasn't doing what it was expected. The upstart status command is
effective at gauging the status of upstart services.