GUI branding support, GUI ssl (HTTPS) support, MTU checking, and OpenVPN bugfixes

1. Regression test passed.
2. GUI Branding: You can change the top-right icon to whatever you want
             (e.g. IBM, or SPEC, or whatever....)
3. GUI SSL: You can provide your own SSL key and certificate for encryption
4. MTU Checking: Some clouds networks have different MTU sizes (or broken MTUs
   in the case of misconfigured VLANs), preventing CB from operating correctly.
   This provides options to override the MTU inside the Guest so that it can
   be reachable.
5. Some bugfixes for OpenVPN support.
6. And some other bugfixes I probably forgot about.