Untrack vagrant_variables.yml and use a sample file

Now to enable the variable file and modify it as much as you want
simply:

git clone ...
cp vagrant_variables.yml.sample vagrant_variables.yml

Edit vagrant_variables.yml

Fixes: #241

Signed-off-by: Sébastien Han <sebastien.han@enovance.com>

README.md

@@ -7,6 +7,7 @@ Clone me:
 
 ```bash
 git clone https://github.com/ceph/ceph-ansible.git
+cp vagrant_variables.yml.sample vagrant_variables.yml
 ```
 
 ## What does it do?