ConfigMap mounts Nexus property directory with wrong permissions on OCP

[ricardozanini]

Describe the bug
The CM for nexus.properties (#212) mounts on $data-dir/etc, which makes the path owned by root. This way the nexus user in the community image can't write the logs in the $data-dir/etc/logback. See #214 (comment) for context.

One way to avoid this is creating an empty volume on $data-dir/etc/logback, which can be handled by the operator. This volume can also be configured to be an actual volume so users can externalize the log data. This is a workaround until we find a solution that avoids the CM being mounted with the wrong permissions.

To Reproduce
Simply deploy the operator on OpenShift.

Expected behavior
That the permissions in the etc directory were set correctly.

Environment
OpenShift 4.x, Nexus Operator 0.6.0

Additional context
See #214 (comment) for context.