FYI  I was able to easily and immediately reproduce the nova-compute/0 hook error by deploying the nova-compute disco-stein functional test in my bastion on our dev cloud (serverstack):

2019-09-05 17:08:34 DEBUG worker.uniter.jujuc server.go:182 running hook tool "juju-log"
2019-09-05 17:08:34 INFO juju-log Setting up the apparmor profile for usr.bin.nova-compute in enforce mode.
2019-09-05 17:08:34 DEBUG config-changed Setting /etc/apparmor.d/usr.bin.nova-compute to enforce mode.
2019-09-05 17:08:35 DEBUG config-changed mkfs.xfs: cannot open /dev/vdb: Device or resource busy
2019-09-05 17:08:35 DEBUG config-changed Traceback (most recent call last):
2019-09-05 17:08:35 DEBUG config-changed   File "/var/lib/juju/agents/unit-nova-compute-0/charm/hooks/config-changed", line 751, in <module>
2019-09-05 17:08:35 DEBUG config-changed     main()
2019-09-05 17:08:35 DEBUG config-changed   File "/var/lib/juju/agents/unit-nova-compute-0/charm/hooks/config-changed", line 744, in main
2019-09-05 17:08:35 DEBUG config-changed     hooks.execute(sys.argv)
2019-09-05 17:08:35 DEBUG config-changed   File "/var/lib/juju/agents/unit-nova-compute-0/charm/hooks/charmhelpers/core/hookenv.py", line 914, in execute
2019-09-05 17:08:35 DEBUG config-changed     self._hooks[hook_name]()
2019-09-05 17:08:35 DEBUG config-changed   File "/var/lib/juju/agents/unit-nova-compute-0/charm/hooks/charmhelpers/contrib/openstack/utils.py", line 1560, in wrapped_f
2019-09-05 17:08:35 DEBUG config-changed     stopstart, restart_functions)
2019-09-05 17:08:35 DEBUG config-changed   File "/var/lib/juju/agents/unit-nova-compute-0/charm/hooks/charmhelpers/core/host.py", line 741, in restart_on_change_helper
2019-09-05 17:08:35 DEBUG config-changed     r = lambda_f()
2019-09-05 17:08:35 DEBUG config-changed   File "/var/lib/juju/agents/unit-nova-compute-0/charm/hooks/charmhelpers/contrib/openstack/utils.py", line 1559, in <lambda>
2019-09-05 17:08:35 DEBUG config-changed     (lambda: f(*args, **kwargs)), __restart_map_cache['cache'],
2019-09-05 17:08:35 DEBUG config-changed   File "/var/lib/juju/agents/unit-nova-compute-0/charm/hooks/charmhelpers/contrib/hardening/harden.py", line 93, in _harden_inner2
2019-09-05 17:08:35 DEBUG config-changed     return f(*args, **kwargs)
2019-09-05 17:08:35 DEBUG config-changed   File "/var/lib/juju/agents/unit-nova-compute-0/charm/hooks/config-changed", line 264, in config_changed
2019-09-05 17:08:35 DEBUG config-changed     configure_local_ephemeral_storage()
2019-09-05 17:08:35 DEBUG config-changed   File "/var/lib/juju/agents/unit-nova-compute-0/charm/hooks/nova_compute_utils.py", line 1015, in configure_local_ephemeral_storage
2019-09-05 17:08:35 DEBUG config-changed     mkfs_xfs(dev, force=True)
2019-09-05 17:08:35 DEBUG config-changed   File "/var/lib/juju/agents/unit-nova-compute-0/charm/hooks/charmhelpers/contrib/storage/linux/utils.py", line 128, in mkfs_xfs
2019-09-05 17:08:35 DEBUG config-changed     check_call(cmd)
2019-09-05 17:08:35 DEBUG config-changed   File "/usr/lib/python3.7/subprocess.py", line 347, in check_call
2019-09-05 17:08:35 DEBUG config-changed     raise CalledProcessError(retcode, cmd)
2019-09-05 17:08:35 DEBUG config-changed subprocess.CalledProcessError: Command '['mkfs.xfs', '-f', '-i', 'size=1024', '/dev/vdb']' returned non-zero exit status 1.
2019-09-05 17:08:35 ERROR juju.worker.uniter.operation runhook.go:132 hook "config-changed" failed: exit status 1
2019-09-05 17:08:35 DEBUG juju.machinelock machinelock.go:180 machine lock released for uniter (run config-changed hook)