Default perm 0600 on /var/lib/mender-configure/device-config.json

Like title says we should restrict the permissions to this core file by Mender Configure (currently it is world-readable with 0644). The reasons:

• There might be sensitive info in it, depending on use. It will be with the upcoming Azure IoT integration (device key)
• Do not think it will create backward-compat issues as the deviceconfig scripts are run as root also

Acceptance criteria

• With all (default) installations, /var/lib/mender-configure/device-config.json has permissions 0600