Support routers without an external gateway

Sometimes we may want to create routers that do not have an external
gateway. This was possible before the router workaround was applied in
0ed105f.

This change makes it possible to define a router without a 'network'
attribute.

Author: markgoddard

roles/os_networks/tasks/router_workaround.yml

@@ -16,6 +16,7 @@
     cloud: "{{ os_networks_cloud | default(omit) }}"
     interface: "{{ os_networks_interface | default(omit, true) }}"
   register: _networks_query
+  when: item.network is defined
 
 - name: Ensure router is registered with neutron
   openstack.cloud.router:
@@ -26,7 +27,7 @@
     interface: "{{ os_networks_interface | default(omit, true) }}"
     name: "{{ item.name }}"
     interfaces: "{{ item.interfaces | default(omit) }}"
-    network: "{{ _networks_query.networks[0].id }}"
+    network: "{{ _networks_query.networks[0].id if _networks_query is not skipped else omit }}"
     external_fixed_ips: "{{ item.external_fixed_ips | default(omit) }}"
     project: "{{ item.project | default(omit) }}"
     state: "{{ item.state | default(omit) }}"