avocado.test: Add missing docstrings

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado.test: Simplify Test.get_tagged_name interface

Test name and test tag can be derived from the class
attributes. Keeping the logdir param is still useful,
as we might want to unittest this interface in the
future.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Merge pull request #35 from avocado-framework/docstrings_small_fixes

Docstrings small fixes