BigSwitch: Add SSL Certificate Validation

This patch adds the option to use SSL certificate
validation on the backend controller using SSH-style
sticky authentication, individual trusted
certificates, and/or certificate authorities.
Also adds caching of connections to deal with
increased overhead of TLS/SSL handshake.

Default is now sticky-style enforcement.

Partial-Bug: 1188189
Implements: blueprint bsn-certificate-enforcement
Change-Id: If0bab196495c4944a53e0e394c956cca36269883
Merge "BigSwitch: Add SSL Certificate Validation"