The installLiberty task can be UP-TO-DATE when no image is installed

[aguibert]

Occasionally I run into issues where the install task thinks it's up to date but there is no wlp image installed. I've ran into two different flavors of this issue:

1. I've attempted to do a ./gradlew clean libertyStart, but the entire wlp dir can't be deleted sucessfully because there is still a server process holding a lock on ws-server.jar which leaves a single file at wlp/bin/tools/ws-server.jar and everything else gets deleted. Since the install task sees the wlp dir, it thinks that there is a full liberty install present.
2. I've written some custom gradle tasks that copy a JDBC driver into my server dir. At first, these copy tasks would run early on in my gradle build, causing creation of wlp/usr/servers/<myserver>/jdbc/driver.jar. This would also result in the liberty install task skipping, because the wlp dir was present. I was able to work around this by saying my copy task mustRunAfter installLiberty, but ideally I wouldn't have to do that.

Currently the installLiberty task computes up-to-date as follows:

        outputs.upToDateWhen {
            getInstallDir(project).exists() && project.buildDir.exists() && new File(project.buildDir, 'liberty-plugin-config.xml').exists()
        }

but I think we should make this a little more specific than the install dir existing. Perhaps also checking that wlp/lib/ws-launch.jar, or some other kernel-type file that will be part of any install exists.