Adding description for testcases - network part3

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: I5f4cd174a3dc2f23188b72ff856ec4e8f4b51414
partially-implements: blueprint testcase-description
8 files changed