903388 - fix service-wait script

wget was not recovering from 'Unable to establish SSL connection.'
type of errors. That caused the script exited with success although
the service was not ready yet (the sleep timeout set for 5 seconds was
not enough).

The fix is to use curl to double check the response for test url (curl is
able to retry form SSL issues).

Also the script exists with non-zero when the test url can't be
reached: when the script succeeds after start, we want to be sure that
the service is really up and running. Otherwise it should fail.

The curl fix should also make the `sleep $ADDITIONAL_TIME` useless (as
this was probably meant to fix the original issue with SSL). Removing,
to eliminate magic constants and speed up the script a bit.