Update contrib/azurerm/README.md (#6057)

The ansible-playbook needs to ssh-login to Azure virtual machines with
ssh keypair, and users need to specify ssh_public_keys for their own
ssh public key. The change of ssh_public_keys is mandatory.
So this updates contrib/azurerm/README.md to explain that.
In addition, the path of all.yml was wrong. That also is updated with
this.