Fixes Jenkins.SYSTEM_READ support

[Vlatombe]

This fixes readonly configuration support by changing multiple checks to Jenkins.ADMINISTER to return an empty validation if permission is missing.

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[daniel-beck]

It doesn't really benefit anyone to show errors to low-privilege users for use cases that are unexpected, so a simpler if (!hasPermission(ADMINISTER)) return FormValidation.ok() replacing the current checkPermission call should be enough. This PR as is should resolve the specific issue encountered too, of course.

[mwebber]

Reported as JENKINS-70987

I am running Jenkins 2.387.1 (latest LTS). When I upgraded the ec2-plugin from 2.0.6 to 2.0.7, I found that all Agents terminated after 10 minutes, which is our Idle termination time, even if a job was running on the Agent. In all cases, Agents were terminated 10 minutes after starting.

When we reverted back to 2.0.6, the problem went away, so this looks like a bug introduced in 2.0.7.
There were 2 PRs in that release, this one #825, and #827.

[mwebber]

In response to the problem I reported in the previous comment (Agents terminating prematurely), deleting and recreating the cloud definition fixes the problem.
See #827 (comment)