Updated docker SSL behaviour for 4.0 onwards

[jennyowen]

SSL instructions were not working for 4.0 so I've updated the behaviour of the /ssl mounted folder checks. I also realised that if a folder is mounted to any of the expected mount points, we should be checking if it's readable before starting neo4j, otherwise it'll fail when trying to read. These checks had been wrapped with the unrelated secure_mode_enabled flag, which should only be used if we are going to do a chown on the mounted folder.

changelog: [4.0, 4.1, packaging, docker]

[eastlondoner]

If we're magically doing configuration we should also consider the following settings: dbms.connector.bolt.tls_level -> should we switch this to REQUIRED dbms.connector.http.enabled -> should we disable this if https is available