Allow endpoints to be disabled in the haproxy_default_services
dict.
This is needed specifically for https://review.openstack.org/#/c/340175/
which requires that the nova-metadata-api service move to the
compute nodes when the networking-calico neutron plugin is used.
In such an environment, the metadata nodes serve requests only for
the local hypervisor and LB endpoints for the metadata service
are not needed.
The corresponding flag disabling haproxy_service_enabled will be
added to the Calico change.
Change-Id: I584fe2647ba0d6a70908e55168360752a08261c5
89f7570c83