add calico VXLAN mode, update docs and vars in sample inventory (#5731)

* calico VXLAN mode

* check vars if calico backend defined

roles/network_plugin/calico/defaults/main.yml

@@ -9,6 +9,8 @@ calico_ipv4pool_ipip: "Off"
 # Use IP-over-IP encapsulation across hosts
 ipip: true
 ipip_mode: "{{ 'Always' if ipip else 'Never' }}"  # change to "CrossSubnet" if you only want ipip encapsulation on traffic going across subnets
+calico_ipip_mode: "{{ ipip_mode }}"
+calico_vxlan_mode: 'Never'
 
 calico_cert_dir: /etc/calico/certs