Added support for slave servers
Add support for master-slave configuration

This also changes the use of defining the "dns" class inside the dns::zone defined type.
Instead the dns class is included and the parameters "zonefilepath" and "namedservicename" pull their values from
the dns class if left as undef.

Initial work came from PR #24.  This varies from that pull request in the following ways

* allow-transfer is not inside the "if master" condition
* A few syntax errors corrected
* Use "join" to generate values for allow-transfer and masters