Adding description for testcases - identity part4

When Tempest is used in customer site, often we are required to
provide a testcase list including testcase names and descriptions.
Now no this kind of doc is available, so we can add descriptions
with the format of doc string for every testcase, so later we
can generata such a testcase description list.

There are hundreds of testcases missing descriptions, so we can
add them gradually, and limit the modified files in one patch
for the convenience of reviewing.

Change-Id: Ia6c3ba8e7ff220be0b0b9bcd64ae2680bec75c4a
partially-implements: blueprint testcase-description
9 files changed