WL-3665 : Don't NPE when PDA Home tool selected

What is currently happening is that the Home tool ("sakai.iframe.site") is being created with a config that has an entry of 'portlet-context=/sakai-web-portlet' in it.  

(This happens in PortletTool.java, line 140: m_finalConfig.setProperty(PortalService.TOOL_PORTLET_CONTEXT_PATH, appName);)


Then, when you later click on a tool on the pda site it checks if the tool has an entry for portlet-context and if it does it creates a toolContextPath (from which it creates the resetActionUrl), but if it doesn't it will NPE when trying to create the resetActionUrl (PDAHandler.java, line 344).

This change says to create the toolContextPath if the tool has an entry for portlet-context or is the Home tool.