Liveness test doesn't fail when rest service is not running

[t-d-d]

The liveness probe is not always failing when it should, specifically when the service seems to be totally down. I think below line should also check for the curl command having failed, e.g. add || [ $? != 0 ]

https://github.com/Landoop/dockers/blob/93f36ff9fb9fcac64fa1038f1cfa48effbcdc7bd/kafka-connect/liveliness.sh#L18

(Also, currently line 24 and 33 syntax error in this case, which doesn't cause a probe failure. Should really be using [[ ]] ?)

[andrewstevenson]

@t-d-d thanks. We will be revamping the images soon. Do you mind submitting a PR?