nova: Use cinder user for Ceph

In Ussuri, nova stopped using separate Ceph keys for the volumes and vms
pools by default. Instead, we set ceph_nova_keyring to the value of
ceph_cinder_keyring by default, which is ceph.client.cinder.keyring.
This is in line with the Ceph OpenStack integration guide [1]. However,
the user used by nova to access the vms pool (ceph_nova_user) defaults
to nova, meaning that nova will still try to use a
ceph.client.nova.keyring, which probably does not exist. We did not see
this issue in CI, because we set ceph_nova_user to cinder.

This change fixes the issue by setting ceph_nova_user to the value of
ceph_cinder_user by default, which is cinder.

Closes-Bug: #1934145
Related-Bug: #1928690

[1] https://docs.ceph.com/en/latest/rbd/rbd-openstack/

Change-Id: I6aa8db2214e07906f1f3e035411fc80ba911a274