Allow overridding LIBMESH_DIR in update_and_rebuild_libmesh.sh

This commit allows the user to set LIBMESH_DIR in their environment and have it honored
when running update_and_rebuild_libmesh.sh. This will allow multiple installed
libmesh versions at the same time. Note that this script will not automatically
delete the contents of LIBMESH_DIR when set for the user's safety.

closes #4431