avocado.core.data_dir: Refactor and introduce .get_data_dir()

Introduce data_dir.get_data_dir(), an API to retrieve the
value of the data dir, a place where we should store data
dir files used by tests, such as ISO files, VM image files,
reference jpeg/png files and the such.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado.plugins: Add core plugin 'datadir'

Add a plugin that can list data directories used by
avocado. This way, we can easily glance what is being
used according to the settings available.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
.travis.yml: Remove unneeded deps, add flexmock

It'll be necessary for the data_dir unittest.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
unittests: Add datadir_unittest

Add one meager unittest that verifies part of the intended
behavior for the data_dir module.

This unittest requires flexmock, that was added to the
travis job in the previous commit.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
docs: Add data_dir documentation

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Merge pull request #39 from avocado-framework/data-dir

Data dir