stork-agent cannot read bind config files that have 600 persmissions

@garbled1 reported in #243:

This was what told me there was an error:
May  9 06:05:35 pollux stork-agent[28307]: #33 (closed)[33mWARN#033[0m[2020-05-09 06:05:35]            bind9.go:254   cannot parse BIND 9 config file /etc/bind/named.conf: exit status 1

At the top of that file, I include /etc/bind/rndc.key
Easy fix was to edit /etc/group:
bind:x:113:stork-agent

stork-agent could be added to bind group during installation if this group exists