[Openstack] A guide on how to replace the in-tree cloudprovider with the external one (#5741)

* add documentation for how to upgrade to the new external cloud provider

* add migrate_openstack_provider playbook

* fix codeblock syntax highligth

* make docs for migrating cloud provider better

* update grammar

* fix typo

* Make sure the code is correct markdown

* remove Fenced code blocks

* fix markdown syntax

* remove extra lines and fix trailing spaces