Query prefixes once for all hosts

And group them into vrf_prefixes for VLAN networks and bgp_prefixes for
servers plugged directly into fabric.

This should reduce the number of queries to NetBox when configuring
firewalls and exit switches. Not sure but I think set_fact helps to
avoid queries (as opposed to setting group_vars).

setup.yml

@@ -1,3 +1,8 @@
+- hosts: '*'
+  roles:
+    - facts
+
+# Set up fabric.
 - hosts: spine-*
   roles:
     - spine
@@ -10,11 +15,13 @@
   roles:
     - exit
 
+# Set up access switches.
 - hosts: access-*, sw-*
   gather_facts: false
   roles:
     - access
 
+# Set up firewall.
 - hosts: fw-*
   roles:
     - firewall