fix util.which if PATH is not in environment

This fixes a test case that failed because PATH was unset in the os.environ.

cloudinit/util.py

@@ -1850,7 +1850,7 @@ def which(program):
         if is_exe(program):
             return program
     else:
-        for path in os.environ["PATH"].split(os.pathsep):
+        for path in os.environ.get("PATH", "").split(os.pathsep):
             path = path.strip('"')
             exe_file = os.path.join(path, program)
             if is_exe(exe_file):