tickets/DM-1062: enable in-place upgrade of EUPS

Enable in-place upgrade of EUPS installed with bin/deploy:
* backwards compatibility: check if EUPS has been installed in $LSSTSW/eups, move it to $LSSTSW/eups/legacy
* install EUPS to $LSSTSW/eups/$EUPS_VERSION
* symlink "$LSSTSW/eups/current' to '$LSSTSW/eups/$EUPS_VERSION"