Ensure that graphadmin schema-create-job exits with a non-zero exit code in case...

Ensure that graphadmin schema-create-job exits with a non-zero exit code in case of failure

- currently, when the GenTester program fails, it is not terminating with a non-zero exit code
- the consequence of this is that the initContainer/job that executes this is stuck and waits forever
- this changes the code in case of failure, which in turn should cause a retry for the container
- also bump version to 1.15.6-SNAPSHOT

Issue-ID: AAI-4136
Change-Id: Ic8a26df66f2ae1a55a1547b562cc6f042f5a513b
Signed-off-by: Fiete Ostkamp <Fiete.Ostkamp@telekom.de>