QA: Convert nodes to objects

Up until now nodes 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 nodes in QA named “_QaNode”. 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.AcquireNode” are also included.

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