The CredentialsTest was suffering intermittent failures with a report that:

1) Error injecting constructor, java.security.KeyStoreException: JENKINS-41987: no X509Certificate found; perhaps instance-identity module is missing or too old
  at jenkins.slaves.JnlpSlaveAgentProtocol4.<init>(JnlpSlaveAgentProtocol4.java:101)

Caused by: java.security.KeyStoreException: JENKINS-41987: no X509Certificate found; perhaps instance-identity module is missing or too old
	at jenkins.slaves.JnlpSlaveAgentProtocol4.<init>(JnlpSlaveAgentProtocol4.java:105)
	at jenkins.slaves.JnlpSlaveAgentProtocol4$$FastClassByGuice$$b470638c.newInstance(<generated>)
	at com.google.inject.internal.cglib.reflect.$FastConstructor.newInstance(FastConstructor.java:40)
	at com.google.inject.internal.DefaultConstructionProxyFactory$1.newInstance(DefaultConstructionProxyFactory.java:61)
	at com.google.inject.internal.ConstructorInjector.provision(ConstructorInjector.java:105)
	at com.google.inject.internal.ConstructorInjector.construct(ConstructorInjector.java:85)
	at com.google.inject.internal.ConstructorBindingImpl$Factory.get(ConstructorBindingImpl.java:267)
	at com.google.inject.internal.ProviderToInternalFactoryAdapter$1.call(ProviderToInternalFactoryAdapter.java:46)
	at com.google.inject.internal.InjectorImpl.callInContext(InjectorImpl.java:1103)
	at com.google.inject.internal.ProviderToInternalFactoryAdapter.get(ProviderToInternalFactoryAdapter.java:40)
	... 14 more

The most recent versions of instance-identity require a Jenkins 2 base
version.  Since this plugin is still based on Jenkins 1.625, it cannot use
the most recent instance-identity.  The 1.5.1 release of instance-identity
seems to be the latest version which supports Jenkins 1.625.

Because the failure is intermittent, I'm not yet confident this resolves
the problem.  Tests will run on this commit in various environments to
see if it helps.