the name of constructed records should be unique so use the slave
reference instead of the "origin" parameter to be sure it is unique.

Before this change, if 2 slaves have the same origin parameter
"foo.com", we have the following in zone-files.yml:

    [...]
    sa.continent.foo.com:
      - cname: cname.of.slave1.for.sa
    [...]
    sa.continent.foo.com:
      - cname: cname.of.slave2.for.sa

So only the cname for slave2 will be used.