add cronjob scenario

[qiliRedHat]

1. Add cronjob type of workload https://issues.redhat.com/browse/OCPQE-13863
2. Add example config section for cronjob type of worklaod to the example configuration file and README
3. Pass USER to the shell_task as env variables, so the shell script can make use of USER such as creating identical namespaces.
4. Show task name as the script name for shell_task type of tasks.

[qiliRedHat]

@svetsa-rh Could you help to review this one? Test results: https://redhat-internal.slack.com/archives/C0266JJ4XM5/p1682569456587699 Test with 6 dev-cronjob, 10 cronjobs for each user, scheduled every 1 minute. Without other workload.

[2023-05-04 04:29:54 UTC] @Qiujie Li - qili,  Reliability test results:
[Function]               |     Total|    Passed|    Failed|Failure Rate|
-----------------------------------------------------------------------
[new_project]            |         6|         6|         0|      0.0%|
[cronjob.sh -n 10]       |         6|         6|         0|      0.0%|
[cronjob.sh -c]          |      5479|      5473|         6|      0.1%|
[cronjob.sh -d]          |         6|         6|         0|      0.0%|
-----------------------------------------------------------------------

[qiliRedHat]

@svetsa-rh I've removed the do-not-merge

[svetsa-rh]

@qiliRedHat What do you mean by "Without other workload."? Which "other workload" are you referring to here?

[svetsa-rh]

@qiliRedHat Also, not sure why the PR is showing still as "Draft".

[svetsa-rh]

/lgtm