refactor(proxmox_lxc_provision): rename password vars and parameterize user name

Rename lxc_password to lxc_root_password for consistency with the new
lxc_user_password (replaces the previously bare 'password' variable in
post-clone.yml, which silently collided with any same-named caller var).
Add lxc_user_name (default: admin) so the non-root account managed in
post-clone.yml is no longer hardcoded. Apply default(omit) to the root
password in create.yml so it is genuinely optional as documented.

BREAKING CHANGE: callers passing lxc_password or a bare 'password' var
must rename to lxc_root_password and lxc_user_password respectively.

roles/proxmox_lxc_provision/tasks/post-clone.yml

@@ -2,16 +2,16 @@
 - name: Change root password
   ansible.builtin.user:
     name: root
-    password: "{{ lxc_password | password_hash('sha512') }}"
+    password: "{{ lxc_root_password | password_hash('sha512') }}"
     update_password: always
-  when: lxc_password is defined
+  when: lxc_root_password is defined
 
-- name: Change admin password
+- name: Change user password
   ansible.builtin.user:
-    name: admin
-    password: "{{ password | password_hash('sha512') }}"
+    name: "{{ lxc_user_name }}"
+    password: "{{ lxc_user_password | password_hash('sha512') }}"
     update_password: always
-  when: password is defined
+  when: lxc_user_password is defined
 
 - name: Regenerate SSH host keys
   ansible.builtin.include_role: