Add case_name as constructor arg

Add case_name as constructor arg

It allows managing multiple TestCase names with only one TestCase
module. It is mainly required by odl which implements:
  - odl,
  - odl_netvirt,
  - fds.

It also renames case to case_name in Features to conform with
TestCases.

JIRA: FUNCTEST-762

Change-Id: Ie254f754a0ea3077a8afda1c470528d38c79478f
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>