Allow aliasing a service so that it's available under more than one name

[mwiencek]

We currently make use of hard-coded service names in consul template files, e.g. {{if service "postgres-master"}}, {{if service "pgbouncer-slave"}}, etc.

Since these map to specific nodes and are a pain to update when we perform a switchover, an haproxy service will be added that detects the current primary and standbys. It will expose the following services: haproxy-postgres-primary, haproxy-postgres-standby, haproxy-postgres-any.

It would be nice if we could keep the new service naming convention while also continuing to allow access to a postgres-master service (which would map to haproxy-postgres-primary). This way existing consul template files will continue to work, and can be updated incrementally without any downtime.

[zas]

For reference:

• https://stackoverflow.com/questions/58274182/how-to-set-consul-alias-service
• https://developer.hashicorp.com/consul/api-docs/agent/check#aliasservice