Add overridable flag to servlet metadata

To resolve https://bugzilla.redhat.com/show_bug.cgi?id=1094248, I believe we need an additional 'overridable' flag on the servlet metadata which indicates whether ServletContainerInitializers (and other code using servlet dynamic registration) should be allowed to override the servlet. Normally this is not allowed, however JBWeb's internal use of DefaultServlet to handle static resources should be.

This only needs to go into 7, since the Undertow subsystem in Wildfly handles this quite differently, so does not need to pass a flag between components.