Big Switch: Fix SSL version on get_server_cert

The ssl.get_server_certificate method uses SSLv3 by default.
Support for SSLv3 was dropped on the backend controller in
response to the POODLE vulnerability. This patch fixes it
to use TLSv1 like the wrap_socket method.

Closes-Bug: #1384487
Change-Id: I9cb5f219d327d62168bef2d7dbee22534b2e454e
(cherry picked from commit 77e283c94f51e21dcf126a316098c54a7cdfca0f)