Fixes #5198 - converts iterator to seq

Test Plan:
test manually - use latest plugins before and after this patch.
Before app creation should fail, should succeed afterwards

Reviewers: aquamatthias, meichstedt, jenkins

Reviewed By: aquamatthias, meichstedt, jenkins

Subscribers: jenkins, marathon-team

Differential Revision: https://phabricator.mesosphere.com/D526

src/main/scala/mesosphere/marathon/core/plugin/impl/PluginManagerImpl.scala

@@ -42,7 +42,7 @@ private[plugin] class PluginManagerImpl(
       case _ => plugin
     }
     val serviceLoader = ServiceLoader.load(ct.runtimeClass.asInstanceOf[Class[T]], classLoader)
-    val providers = serviceLoader.iterator()
+    val providers = serviceLoader.iterator().toSeq
     val plugins = definitions.plugins.withFilter(_.plugin == ct.runtimeClass.getName).map { definition =>
       providers
         .find(_.getClass.getName == definition.implementation)