Adds a setting for the location of the nzf files

Bind9 stores the nzf files in a configurable path, specified in
named.conf. To make sure designate looks in the right place, this patch
adds support to specify the correct path in designate.conf as well.

Change-Id: I75d3e982379b487a746987c61f9a4e0de8ff56da
Closes-Bug: 1280418
Merge "Adds a setting for the location of the nzf files"