provider/openstack: Update Development Documentation (#12265)
provider/openstack: Update Development Documentation (#12265)

This commit removes the bundled devstack script and updates the
documentation to point to the latest build scripts used for
testing. It also mentions that development should be possible on
any OpenStack environment.
haxorof/terraform-provider-openstack,haxorof/terraform-provider-openstack,haxorof/terraform-provider-openstack