refactor(system_setup): rename bare 'password' var to user_password

The bare 'password' variable in user.yml could silently collide with any
same-named variable elsewhere in scope. Rename to user_password to
namespace it alongside username.

BREAKING CHANGE: callers passing 'password' to this role must rename it
to user_password.

roles/system_setup/tasks/user.yml

@@ -2,7 +2,7 @@
 - name: "Create a new user {{ username }}"
   user:
     name: "{{ username }}"
-    password: "{{ password | password_hash('sha512') }}"
+    password: "{{ user_password | password_hash('sha512') }}"
     groups:
       - sudo
     shell: "{{ shell }}"