Added support for slave servers

Hi,

this change is the only reason why I can't use the upstream version.
Would it be possible to include it into theforeman/puppet-dns?
Thanks in advance.
Clemens
@cbergmann We (the foreman team) treat this module as a full fork so any changes are welcome.

@ohadlevy I doubt that works because ```;``` is not glue, it's trailing after each entry. AFAIK the only valid syntax is ```masters { master1; master2; };```.