Now using SINGULARITY_BIN if defined

SiteTests/testjob/tests/CE-cms-singularity-wrapper

@@ -33,7 +33,8 @@ for LOCATION in \
 done
 export PATH
 
-singularity_locate_bin /usr/bin $OSG_SINGULARITY_IMAGE_DEFAULT
+SINGULARITY_BIN=${SINGULARITY_BIN:-""}
+singularity_locate_bin $SINGULARITY_BIN $OSG_SINGULARITY_IMAGE_DEFAULT
 
 # Grid environment is taken from OSG, check it's available:
 if [ ! -e /cvmfs/oasis.opensciencegrid.org/osg-software/osg-wn-client ]; then