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
Merge "Big Switch: Fix SSL version on get_server_cert"