add SSH PTY for NOKIA SROS (fix for #86)

[vitalisator]

Fixes:

• Fix debug options for SSH
• add basic support for PTY and Nokia based devices fix Nokia-SROS #86
• Support for VRF per router. $config['routers']['router1']['vrf'] = '1234';

[gmazoyer]

Looks good to me. Maybe, for potential other types of router, it would be wise to put this kind of specific code in a dedicated and overridable function. I think that it would be wise and doable, but the Authentication object will need to know about the Router object we are dealing with somehow.

[vitalisator]

yes, that was my thought too. But a I'm not so familiar with OOP stuff and currently structure of looking-glass.
what would be the points to touch?

[gmazoyer]

Looks good to me. I'd like to keep the VRF work part out of this PR though. I think it worth creating another PR to introduce the VRF feature since this is a change that can be applied to multiple router types.