r/hass-dhcp: Clean up DHCP/DNS service

The production deployment of *dnsmasq* for Home Assistant has deviated
from how the *hass-dhcp* role configures it.  Bringing the role back in
sync with how things really are.

roles/hass-dhcp/vars/main.yml

@@ -0,0 +1,9 @@
+hass_net_cidr: >-
+  {{
+    (
+      ansible_facts[hass_interface].ipv4.network
+      + '/'
+      + ansible_facts[hass_interface].ipv4.netmask
+    )
+    | ipaddr('net')
+  }}