fix bug when DZI_SKIP_EXISTING is not set in environment
fix bug when DZI_SKIP_EXISTING is not set in environment

it should default to false (enable overwrite) unless set to 't' or 'true'.

informatics-isi-edu/microscopy,informatics-isi-edu/microscopy,informatics-isi-edu/microscopy,informatics-isi-edu/microscopy