(#16553) Exclude portmap-wait from upstart services

The portmap-wait upstart service is not a service by itself (it's a helper for
the portmap service) and does not respond to the status command, it returns 1
along with an error message. This commit adds portmap-wait to the excludes
method of the init provider, which is also used by the upstart provider to
exclude services from self.instances. This also adds the portmap-wait service
to the upstart provider spec test to verify that it is correctly excluded from
self.instances.