This commit implements the serialized test ids described in the
Introduce proper test IDs RFC.

https://www.redhat.com/archives/avocado-devel/2016-March/msg00024.html

It implements `TestName` class, which contains the test uid, test name
and the variant uid and allows querying for the file-system-friendly
name.

The workflow is:

1. tests are discovered, name is translated to "Test Name" by loader
2. test_suite is passed to the runner (new) along with the number of
   tests+variants to be executed (used to get number of digits)
3. the Mux (params generator) yields the template + (new) variant id
4. the runner replaces template['name'] to TestName(uid, test_name,
   variant_id); where uid is currently no executed tests, test_name is
   the original name from Loader and variant_id is either None or the
   variant index.

This commit makes the `tag` argument unused. To avoid problems a warning
is issued on it's usage so we can remove it in the upcoming releases.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>