RAPI client: Don't re-use PycURL object

With this patch, a new PycURL object will be created for each request.
This should make the RAPI client safe for simultaneous calls from
multiple threads. Unittests are adjusted accordingly.

An unnecessary variable assignment is also removed from the unittest
script.

This patch survived a small QA and unittests.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>