Batch up dynamic plugin installations from setup wizard (#4124)

* Batch up dynamic plugin installations from setup wizard.

* Reworked to thread the list of batched-up plugins properly and introduced CompleteBatchJob to force the setup wizard to wait.

* UpdateCenterPluginInstallTest.test_installKnownPlugins failure.

* @amuniz notes that an existing message could now be considered misleading.

* Introduced UpdateCenter.DownloadJob.getDisplayName for better consistency in logging, as suggested by @Vlatombe.

* Reducing log levels of messages from dynamicLoad which are not of great interest, as suggested by @Vlatombe.

* Reducing log level of another message of little interest.

* Adding classic UI for CompleteBatchJob.

* CompleteBatchJob should be used only when dynamicLoad.

* Null defense on UpdateCenterJob.site.
Otherwise can get:
java.lang.NullPointerException
	at hudson.model.UpdateCenter.getConnectionCheckJob(UpdateCenter.java:850)
	at hudson.model.UpdateCenter.addConnectionCheckJob(UpdateCenter.java:817)
	at hudson.model.UpdateCenter.addJob(UpdateCenter.java:804)
	at hudson.model.UpdateSite$Plugin.deploy(UpdateSite.java:1367)
	at hudson.PluginManager.install(PluginManager.java:1537)
	at hudson.PluginManager.install(PluginManager.java:1496)
	at hudson.PluginManager.doInstall(PluginManager.java:1445)
	at …