Use rand_uuid and remove unused client for clean-up

This patch cleans the code of test_servers_negative.py up with the
following ways:
(1) Remove an unused client in the test file.
(2) Write some test descriptions.
(3) Import test based on http://docs.openstack.org/developer/hacking/
(4) Use data_utils.rand_uuid() instead of str(uuid.uuid4()).
    test_servers_negative contains some tests about nonexistent
    server/flavor and nonexistent uuids are generated with str(
    uuid.uuid4()).
    Now rand_uuid() is defined in data_utils for generating their
    uuids. This patch replaces nonexistent uuids to rand_uuid().

Change-Id: I2fa423f11e7703a69720f031a8462d4eb782d76c
Merge "Use rand_uuid and remove unused client for clean-up"