powerdns: fix for several slaves with same origin

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.