Ensure to decode bytes or fail

The commit fcf289797c introduces the
pattern:

  if isinstance(line, bytes):
    try:
      line = line.decode(encoding='utf-8')
    except UnicodeError:
      pass
  # concat line with a string

which is not working in PY3K if an UnicodeError is raised because line
is not decoded and concatened to a string.

This change delegates decoding to safe_decode[1] which returns a text
object or raises an error.

[1] oslo_utils.encodeutils

Closes-Bug: #1503415
Related-Bug: #1499004
Change-Id: I16b8013f33aa3efad65be8040d3210120e047bbd

neutronclient/tests/unit/test_cli20.py

@@ -73,13 +73,7 @@ class FakeStdout(object):
     def make_string(self):
         result = ''
         for line in self.content:
-            if six.PY3:
-                if isinstance(line, bytes):
-                    try:
-                        line = line.decode(encoding='utf-8')
-                    except UnicodeError:
-                        pass
-            result = result + line
+            result += encodeutils.safe_decode(line, 'utf-8')
         return result