Add support for Jenkins-style 'H' seed job's cron field

[surmabck]

Describe the bug
Currently, it is not possible to configure seed jobs using Jenkins-style 'H' hashed field value, cuz Jenkins Operator's validation is delegating cron expression validation to robfig/cron which obviously does not support it.
'H' expressions are really cool feature of Jenkins which Jenkins native operator should support.

To Reproduce
Declare seed job with

buildPeriodically: 'H/10 * * * *'

and observe operator output

Additional information
This is the line which is causing it

kubernetes-operator/pkg/configuration/user/seedjobs/validate.go

Line 123 in 3a143a0

_, err := cron.Parse(str)

Kubernetes version:
1.21.1
Jenkins Operator version:
0.7.0

Add error logs about the problem here (operator logs and Kubernetes events).
N/A

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. If this issue is still affecting you, just comment with any updates and we'll keep it open. Thank you for your contributions.

[surmabck]

Any update on this?

[prryb]

That validation is gone for now. Released in v0.7.1.