rework conditionals in Set-WindowsUpdateServer

[jeffallen-google]

The conditional on line 76 appears to be intended to prevent re-execution after reboot which could cause a reboot loop, but this conditional is also preventing 1st time execution because the test path already exists in a clean install from Windows Server 2019 ISO. Because this test path already exists, registry values based on wu_server_url and wu_server_port are not being set and it is not possible to use a custom server for Windows updates.

In this pull request, the logic has been changed to remove the conditional based on test path existence and instead to set registry values if the new values are different than the current registry. Further the reboot is only performed if registry values are changed.

[google-oss-prow]

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[EricEdens]

/assign @bkatyl