[processing] use os.path.join instead of + os.sep + (fix #3552)

(partially cherry-picked from 37951ba)

Author: alexbruy

python/plugins/processing/algs/otb/OTBAlgorithm.py

@@ -178,7 +178,7 @@ def processAlgorithm(self, progress):
         path = OTBUtils.otbPath()
 
         commands = []
-        commands.append(path + os.sep + self.cliName)
+        commands.append(os.path.join(path, self.cliName))
 
         self.roiVectors = {}
         self.roiRasters = {}