GPG error - debian nautilus bionic is not signed

Description of the problem:
I think we are missing one step before adding the repository during the plugin installation.

Version:
Ubuntu 18 Bionic

How reproducible:
Always

Steps to Reproduce:
1. Add the line to local.conf
enable_plugin devstack-plugin-ceph https://opendev.org/openstack/devstack-plugin-ceph.git
2. ./stack

Actual results:
++/opt/stack/devstack-plugin-ceph/devstack/lib/ceph:configure_repo_ceph:956 sudo apt-add-repository -y 'deb https://download.ceph.com/debian-nautilus/ bionic main'
Hit:1 http://ar.archive.ubuntu.com/ubuntu bionic InRelease
Hit:2 http://ar.archive.ubuntu.com/ubuntu bionic-updates InRelease
Hit:3 http://ar.archive.ubuntu.com/ubuntu bionic-backports InRelease
Hit:4 http://ubuntu-cloud.archive.canonical.com/ubuntu bionic-updates/train InRelease
Hit:5 http://ar.archive.ubuntu.com/ubuntu bionic-security InRelease
Get:6 https://download.ceph.com/debian-nautilus bionic InRelease [8,539 B]
Err:6 https://download.ceph.com/debian-nautilus bionic InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY E84AC2C0460F3994
Reading package lists...
W: GPG error: https://download.ceph.com/debian-nautilus bionic InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY E84AC2C0460F3994
E: The repository 'https://download.ceph.com/debian-nautilus bionic InRelease' is not signed.
+/opt/stack/devstack-plugin-ceph/devstack/lib/ceph:configure_repo_ceph:1 exit_trap

Expected results:
Install the plugin without problems.

Additional info:
To fix this only add manually the key [1] :
wget -q -O- 'https://download.ceph.com/keys/release.asc' | sudo apt-key add -

[1] https://docs.ceph.com/en/latest/install/get-packages/#apt