Update handling of required objects.

...cAgentConfiguration_requiredObjects_RequiredContainer.gif

...ctool16/CreateRequiredContainer_container_ComputeNode.gif

.../CreateRequiredContainer_container_RuntimeEnvironment.gif

...ctool16/CreateRequiredContainer_container_StorageNode.gif

...agent.generic.edit/icons/full/obj16/RequiredContainer.gif

tools.descartes.prisma.agent.generic.edit/plugin.properties

...ric/genericagent/provider/RequiredObjectItemProvider.java

...s.descartes.prisma.agent.generic/model/genericagent.ecore

...escartes.prisma.agent.generic/model/genericagent.genmodel

...rc/tools/descartes/prisma/agent/generic/GenericAgent.java

...risma/agent/generic/genericagent/GenericAgentPackage.java

...tes/prisma/agent/generic/genericagent/RequiredObject.java

...nt/generic/genericagent/impl/GenericAgentPackageImpl.java

...a/agent/generic/genericagent/impl/RequiredObjectImpl.java