[New feature] Support for SSH-only VMs

[frisso]

In some cases, a console-mode is enough. This would greatly decrease the necessities in terms of CPU on the cluster.
It would be helpful to support also SSH-only backend services, which may require a different "ingress controller" that is able to demultiplex SSH sessions toward the involved VM.

[giorio94]

The basics of this feature have been addressed in #295, #312, #313

Yet, some functionalities are still missing:

• The possibility to connect to the CLI only VMs through the web-based dashboard (e.g. through an emulated terminal);
• The possibility to connect to the CLI only VMs through SSH from outside the cluster (e.g. from home). This has been implemented by the bastion-operator (@claudious96)

Possibly useful pointers:

• Web SSH client: https://github.com/billchurch/WebSSH2
• SSH over websocket: https://github.com/erebe/wstunnel