Add documentation about different variable args which overcloud-deploy role in tqe supports

[Sandeep Yadav]

We have different args[1] which overcloud-deploy role supports(tripleo-quickstart-extras). These args combine to make deploy_args which allows us to make the final deployment command.

We don't have an explanation about them in overcloud-deploy role README.md file[2] or anywhere else.

We use these different args all over in our featureset files[3] and good explanation in overcloud-deploy role will be beneficial.

[1] https://opendev.org/openstack/tripleo-quickstart-extras/src/branch/master/roles/overcloud-deploy/defaults/main.yml#L117-L145

[2] https://opendev.org/openstack/tripleo-quickstart-extras/src/branch/master/roles/overcloud-deploy/README.md

[3] https://codesearch.opendev.org/?q=_args&i=nope&files=config%2Fgeneral_config&excludeFiles=&repos=openstack/tripleo-quickstart