Do not try to call decode() on a text string

This fixes a call to 'isinstance' that only worked on Python2.

Closes-Bug: #1265879
Change-Id: I886da95f1ad2a8314de92be8aecb354ee3124e6d
Prevent dictionary size from changing while iterating over its items

In Python 3, dict.items() returns 'a dict_item'. Iterating over it while
deleting some of the dictionary elements is forbidden. We have to iterate over
a list to avoid getting this error:

    RuntimeError: dictionary changed size during iteration

Change-Id: I43401e6eb9a31148fda4677644bf99e1b739d0dd
Rename using-api.rst to using-api-v2.rst

Renaming file for consistency, since using-api-v3.rst is being written
there: https://review.openstack.org/#/c/63408

Minor changes have been made to indicate that the file documents the
keystone v2 API.

Change-Id: I694b658a8b59d21615af5d88edc0f7b394ebbe7b
Partial-Bug: #1260527
Merge "Prevent dictionary size from changing while iterating over its items"
Merge "Rename using-api.rst to using-api-v2.rst"
Merge "Do not try to call decode() on a text string"