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/update.yml

@@ -3,7 +3,7 @@
   community.proxmox.proxmox:
     vmid: "{{ lxc_vmid }}"
     hostname: "{{ lxc_hostname }}"
-    password: "{{ lxc_password | default(omit) }}"
+    password: "{{ lxc_root_password | default(omit) }}"
     cores: "{{ lxc_cores }}"
     memory: "{{ lxc_memory }}"
     swap: "{{ lxc_swap }}"