Azure: Rename apply-rg_2.sh to apply-rg.sh (#6049)

apply-rg.sh was for Azure command version 1("azure" command) and the
command is old and version 2("az" command) is officially used today.
apply-rg_2.sh was for the version 2. In addition, the README[1] says
we need to run apply-rg.sh for applying templates.

This renames apply-rg_2.sh to apply-rg.sh for common usages of the
version 2.

[1]: https://github.com/kubernetes-sigs/kubespray/tree/master/contrib/azurerm#generating-and-applying