Fix IllegalArgumentException in markExhausted for template-based instance configurations

[sombraSoft]

When a GCE capacity error (ZONE_RESOURCE_POOL_EXHAUSTED) hits an instance-template-based configuration, markExhausted() throws while formatting its log message:

java.lang.IllegalArgumentException
    at com.google.common.base.Preconditions.checkArgument(Preconditions.java:127)
    at com.google.cloud.graphite.platforms.plugin.client.util.ClientUtil.nameFromSelfLink(ClientUtil.java:84)
    at com.google.jenkins.plugins.computeengine.InstanceConfiguration.markExhausted(InstanceConfiguration.java:501)
    at com.google.jenkins.plugins.computeengine.ComputeEngineComputerLauncher.launch(ComputeEngineComputerLauncher.java:209)
    at hudson.slaves.SlaveComputer.lambda$_connect$0(SlaveComputer.java:298)

Template-based configurations leave machineType empty (it is only read when no template is set), and nameFromSelfLink("") rejects empty input. Consequences (hit in production during a real us-central1-a stockout):

• the exception escapes launch() before terminateNode() runs, so the dead node is relaunched in a loop by the retention strategy and Jenkins shows "Unexpected error in launching an agent. This is probably a bug in Jenkins" on every cycle;
• the zone is never marked exhausted, so the Support handling ZONE_RESOURCE_POOL_EXHAUSTED errors by allowing multiple fallback options #555 fallback-zone feature never engages and the provisioner keeps hammering the stocked-out zone.

The fix guards the log message: use machineType when set, fall back to the template name for template-based configurations.

Resolves #561

Testing done

• New unit test InstanceConfigurationTest#testMarkExhaustedDoesNotThrowForTemplateBasedConfiguration builds a configuration with an empty machine type and a template, calls markExhausted, and asserts the zone is marked. It fails before the fix (IllegalArgumentException) and passes after.
• Full InstanceConfigurationTest class passes (28/28).
• Verified in a production Jenkins instance (public OpenROAD CI): with machineType populated as a workaround, markExhausted succeeds and the fallback-zone logic provisioned correctly in another zone during a subsequent real stockout.

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[das7pad]

LGTM, thanks!

(We also see this in our logs at work.)