Fix for non-existing Pixels directory in bin/omero admin cleanse

OME-DCO-1.1-Signed-off-by: Sebastien Besson <seb.besson@gmail.com> (github: sbesson)
Do not restrict existence check to Pixels directory in cleanse command

OME-DCO-1.1-Signed-off-by: Sebastien Besson <seb.besson@gmail.com> (github: sbesson)
Merge pull request #2647 from sbesson/rebased/develop/cleanse_no_pixels

Fix for non-existing Pixels directory in bin/omero admin cleanse (rebased onto develop)