nova-cell role clone failed

when enable kolla_dev_mod, nova-cell role clones code failed,
because we use nova-cell repository which is not exists.
in fact, nova-cell role should use nova repository too

Change-Id: I7fa62726d0d5b0aeb3bd5fa06dc0e59667f94fa0

ansible/roles/nova-cell/defaults/main.yml

 ---
-project_name: "nova-cell"
+project_name: "nova"
 
 nova_cell_services:
   nova-libvirt: