Provide fallback .default-server.xml in case $STASH_HOME/shared/server.xml is missing or unreadable

Since Stash 3.8 the server.xml configuration file is no longer local to the Stash installation directory (and thus not carried over automatically on upgrade to new versions) but is read from $STASH_HOME/shared/server.xml.

If this file is later deleted or made unreadable Stash would fail to start and report an error.

In 3.9.2+ a default .default-server.xml is now created by the Stash Installer and used in case server.xml is missing or unreadable from all of $STASH_HOME, $STASH_HOME/shared, and the Stash installation directory.