Share get_discovery_url in templates
Share get_discovery_url in templates

Move get_discovery_url to BaseTemplateDefinition so swarm and k8s template
can share it.

Behavior changes
Change swarm's discovery member number from 1 to bay.master_count

Partially implements: blueprint swarm-high-availability
Change-Id: I2617bc81daf68b45148987190a2f04820ad18bfd

ArchiFleKs/magnum,openstack/magnum,jay-lau/magnum,openstack/magnum,ArchiFleKs/magnum