Use `six.text_type` instead of `str` in cliutils

Several clients support printing dict with unicode using method
`print_dict`.

Using `unicode()` does not satisfy the Python 3 compatibility, so
we need to use `six.text_type` to allow print dict with unicode and
to support both Python 2 and 3.

Related to bp make-python3-compatible

Change-Id: If87fba3a5a53d2d6a97bd6d2cae6779b7dc81eed
Merge "Use `six.text_type` instead of `str` in cliutils"