Enable NSS wrapper only if /tmp is writable

[sivanov-nuodb]

Issue

The container will fail if readOnlyRootFilesystem=true and no volume is
mounted in /tmp. This is the case in NuoDB Helm charts with
nuocollector-config and backup-hooks containers.

Changes

Enable NSS wrapper only if /tmp is writable.

Notes

The changes are reviewed more easily with show whitespaces checkbox disabled.

[adriansuarez]

Looks good.

The way we deal with this in the main container is that the /tmp directory has an ephemeral volume mounted on it. I wonder if we can do the same thing here, or use the same volume (though there might be a timing issue with which container creates the virtual user and group entries).

But this might not even matter if we only need the uid and gid to match.