make path to nosetests executable configurable

this is to make use python2 when nosetests points to python3:

make NOSETEST=nosetests-2.7 tests

Makefile

@@ -59,12 +59,14 @@ ifeq ($(OFFICIAL),)
 endif
 RPMNVR = "$(NAME)-$(VERSION)-$(RPMRELEASE)$(RPMDIST)"
 
+NOSETESTS := nosetests
+
 ########################################################
 
 all: clean python
 
 tests:
-	PYTHONPATH=./lib nosetests -d -v
+	PYTHONPATH=./lib $(NOSETESTS) -d -v
 
 # To force a rebuild of the docs run 'touch VERSION && make docs'
 docs: $(MANPAGES) modulepages