Add installation of requirements for Azure (#6076)

Due to lack of requirements installation on Azure README, the error
can happen:

 "The ipaddr filter requires python's netaddr be installed on the
  ansible controller"

It is nice to add the installation for Azure users.

contrib/azurerm/README.md

@@ -60,6 +60,7 @@ It will create the file ./inventory which can then be used with kubespray, e.g.:
 
 ```shell
 $ cd kubespray-root-dir
+$ sudo pip3 install -r requirements.txt
 $ ansible-playbook -i contrib/azurerm/inventory -u devops --become -e "@inventory/sample/group_vars/all/all.yml" cluster.yml
 ```