[WIP] Java 11 readiness: build both on JDK8 and JDK11

[batmat]

Work in progress to make sure this plugin is constantly checked both building with JDK8 and JDK11.

This goes with the Jenkins Java 11 General Availability announcement made back in March, and we're making a pass to check plugins are looking good on a JDK11.

(to understand the Jenkinsfile content)

[batmat]

FTR, FindBugs issues on newer baseline, even on JDK8.

Also, locally test failures

[INFO] Tests run: 6, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 16.445 s - in org.jenkinsci.plugins.structs.SymbolLookupTest
[INFO] Running org.jenkinsci.plugins.structs.describable.DescribableModelTest
[ERROR] Tests run: 53, Failures: 5, Errors: 0, Skipped: 0, Time elapsed: 9.021 s <<< FAILURE! - in org.jenkinsci.plugins.structs.describable.DescribableModelTest
[ERROR] parametersDefinitionProperty(org.jenkinsci.plugins.structs.describable.DescribableModelTest)  Time elapsed: 0.019 s  <<< FAILURE!
java.lang.AssertionError: expected:<{parameterDefinitions=[{$class=BooleanParameterDefinition, defaultValue=false, name=flag}, {$class=StringParameterDefinition, name=text}]}> but was:<{parameterDefinitions=[{$class=BooleanParameterDefinition, defaultValue=false, name=flag}, {$class=StringParameterDefinition, name=text, trim=false}]}>
        at org.junit.Assert.fail(Assert.java:88)
        at org.junit.Assert.failNotEquals(Assert.java:834)
        at org.junit.Assert.assertEquals(Assert.java:118)
        at org.junit.Assert.assertEquals(Assert.java:144)
        at org.jenkinsci.plugins.structs.describable.DescribableModelTest.roundTrip(DescribableModelTest.java:894)
        at org.jenkinsci.plugins.structs.describable.DescribableModelTest.roundTrip(DescribableModelTest.java:885)
        at org.jenkinsci.plugins.structs.describable.DescribableModelTest.parametersDefinitionProperty(DescribableModelTest.java:635)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:498)
        at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
        at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
        at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
        at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
        at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
        at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
        at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
        at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
        at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
        at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
        at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
        at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
        at org.junit.rules.ExternalResource$1.evaluate(ExternalResource.java:48)
        at org.jvnet.hudson.test.JenkinsRule$1.evaluate(JenkinsRule.java:556)
        at org.junit.internal.runners.statements.FailOnTimeout$CallableStatement.call(FailOnTimeout.java:298)
        at org.junit.internal.runners.statements.FailOnTimeout$CallableStatement.call(FailOnTimeout.java:292)
        at java.util.concurrent.FutureTask.run(FutureTask.java:266)
        at java.lang.Thread.run(Thread.java:748)

[batmat]

Superseded by #121