Ask and remember which Tomcat servlet path to use when installing the Debian package

Right now the only supported way to install XWiki on Debian is with the /xwiki/ path. It's very easy to configure to have XWiki on root app but then you have to redo it after each upgrade (see https://www.xwiki.org/xwiki/bin/view/Documentation/AdminGuide/Installation/InstallationViaAPT/#HXWikiasrootwebapp28shortURLs29).

It should not be too hard to ask the servlet path the first time and remember it like dbconfig does for the database password.