docs: PowerDNS 4.1 updates
docs: PowerDNS 4.1 updates

The PowerDNS project hosts their latest HTTP API documentation at a
slightly different URL, and they've retired the "json-interface" setting
altogether. Update our pdns server configuration instructions to reflect
these changes.

Closes #1092

vongazman/libcloud,pquentin/libcloud,ZuluPro/libcloud,Kami/libcloud,StackPointCloud/libcloud,mistio/libcloud,andrewsomething/libcloud,Kami/libcloud,Kami/libcloud,Scalr/libcloud,erjohnso/libcloud,apache/libcloud,Scalr/libcloud,vongazman/libcloud,vongazman/libcloud,andrewsomething/libcloud,ZuluPro/libcloud,Scalr/libcloud,StackPointCloud/libcloud,andrewsomething/libcloud,ByteInternet/libcloud,ByteInternet/libcloud,StackPointCloud/libcloud,apache/libcloud,erjohnso/libcloud,erjohnso/libcloud,pquentin/libcloud,apache/libcloud,pquentin/libcloud,mistio/libcloud,ZuluPro/libcloud,ByteInternet/libcloud,mistio/libcloud