file is optional for slave zones

[ryanwalder]

In named.conf.local file is optional see here.

Due to the way the formula is written at present this means we need to define an entire site in available_zones just to add the file directive to named.conf.local rather than just skipping writing the file directive if not specified.

I've put a bodge into my local copy to get round my pressing issue, but need to look into a sane way of doing it in the macro in files/named.conf.local.jinja.