external ceph does not copy per-node ceph.conf

When using an advanced per-node configuration, the "Copy over ceph.conf" task of nova-cell/tasks/external_ceph.yml only checks for a ceph.conf at the "{{ node_custom_config }}/nova/ceph.conf" when "{{ node_custom_config }}/nova/{{ inventory_hostname }}/ceph.conf" should also a perfectly valid option.

**Bug Report**

What happened:
TASK [nova-cell : Copy over ceph.conf] ************************************************************************************************************************************************
failed: [ip/hostname] (item=nova-compute) => {"ansible_loop_var": "item", "changed": false, "item": "nova-compute", "msg": "Could not find or access '/etc/kolla/config/nova/ceph.conf' on the Ansible Controller.\nIf you are using a module and expect the file to exist on the remote, see the remote_src option"}

What you expected to happen:
This copy should have passed as all hosts provide the ceph.conf file in their host specific folder under {{ node_custom_config}}/nova/{{ inventory_hostname }}/ceph.conf

How to reproduce it (minimal and precise):
Utilize only {{ node_custom_config}}/nova/{{ inventory_hostname }}/ceph.conf to provide kolla-ansible with ceph.conf file nova expects

**Environment**:
* OS (e.g. from /etc/os-release): VERSION="18.04.4 LTS (Bionic Beaver)"
* Kernel (e.g. `uname -a`): Linux 4.15.0-101-generic #102-Ubuntu SMP Mon May 11 10:07:26 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux
* Docker version if applicable (e.g. `docker version`): 19.03.11
* Kolla-Ansible version (e.g. `git head or tag or stable branch` or pip package version if using release): stable/train but this affects master as well
* Docker image Install type (source/binary): binary
* Docker image distribution: ubuntu
* Are you using official images from Docker Hub or self built? Docker Hub
* If self built - Kolla version and environment used to build:
* Share your inventory file, globals.yml and other configuration files if relevant