Use python-openssl for https connections

Update VerifiedHTTPSConnection to use python-openssl for its connect method.

In Python 2.7 there are limited ssl features.

Using python-openssl would allow:

* The ability to disable SSL compression on a per connection basis for performance reasons. With compressed qcow2 images the download performance can increase by more than 400% when the redundant compression is disabled.
* Neater loading of the default system CA cert files.

Note that much of the functionality in python-openssl maps to Python 3.3 so if/when we move to a newer version of Python replacing the python-openssl code with Python 3.3 code should be simple.