Add support for overrides context for portlet contexts

[bjagg]

uPortal-start promotes the use of overridesContext.xml usage to customize Spring beans. JasigWidgetPortlets does not currently support this -- it's the only standard portlet that does not. This requires customization to context files to copy and overwrite the original context in a location different than expected.

Solution is to add the overrides context to each portlet defintion in portlet.xml.

For example, here is the change for SimpleJspPortlet:

        <init-param>
            <name>contextConfigLocation</name>
            <value>/WEB-INF/context/portlet/simpleJspPortletContext.xml, 
                   classpath:/properties/contextOverrides/simpleJspPortletOverridesContext.xml</value>
        </init-param>

Also, web.xml should be updated to load overridesContext.xml

	<context-param>
		<param-name>contextConfigLocation</param-name>
		<param-value>/WEB-INF/context/applicationContext.xml,
                             classpath:/properties/contextOverrides/overridesContext.xml</param-value>
	</context-param>

Default "empty" overrides context files may need to be added into src/main/resources/properties/contextOverrides/. See https://github.com/Jasig/WebproxyPortlet/blob/master/src/main/resources/properties/contextOverrides/overridesContext.xml as an example.