Improve readme to include all required vars and optional vars

Closes #7 (closed)

roles/configure-bes/tasks/main.yml

 ---
+- name: Assert that we have a ssh_key_users
+  assert:
+    that: ssh_key_users is defined
+    msg: "ssh_key_users needs to be defined as the list of users whom public keys need to be installed"
+
 - name: Assert that we have a bes_path_to_pub_keys
   assert:
     that: bes_path_to_pub_keys is defined