Use pyOpenSSL for HTTPS connections.
This allows:
* Neater loading of system CA files
* Optional disabling of SSL compression
The performance gain from disabling SSL compression is significant
in cases where the image being uploaded/downloaded is in an already
compressed format (eg qcow2).
Related to bp ssl-connect-rework.
Change-Id: I0568b6c95c5fc7b8eafdbd0284e24c453660a55a