QA: Convert instances to objects

Up until now instances were stored as a dictionary. The keys were
hardcoded in a lot of places and entries modified directly.

This patch introduces a new class for instances in QA named
“_QaInstance”. It still supports accessing details via dictionary
syntax, but that will be removed after a couple of patches changing all
users. Unit tests for “qa_config.AcquireInstance” are also included.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Bernardo Dal Seno <bdalseno@google.com>