Use to_utf8() instead of safe_encode() in convert_str()

safe_encode() convert string using locale [1]. To avoid
possible confuses we should use to_utf8() method instead.

[1] https://wiki.openstack.org/wiki/Python3#safe_encode

Change-Id: I4b8d81bcec389f224084e5270e884e38836d1136

cinder/utils.py

@@ -831,7 +831,7 @@ def convert_str(text):
     * convert to Unicode on Python 3: decode bytes from UTF-8
     """
     if six.PY2:
-        return encodeutils.safe_encode(text)
+        return encodeutils.to_utf8(text)
     else:
         if isinstance(text, bytes):
             return text.decode('utf-8')