avocado.runner: Fix typo in test stream

We're looking to replace `stderr`, not `sterr`.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado: Mass import cleanup

Make non standard python modules to be imported after
standard ones, also respect lines between imports and
the rest of the code. Also, remove unused imports.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Merge pull request #307 from lmr/refactor-cleanups

Refactor cleanups