Add support for ietf-ssh-server YANG

[troglobit]

The draft IETF model for SSH provides a grouping that can be included in the infix services model.

Scope:

• [ ] Enable disable service
• [ ] Change listening port
• [ ] Bind to a single interface
• [ ] Host key similar to NETCONF "genkey" setup

Future:

• [ ] Add support for certificates

Note: netopeer2-server use libssh for access on port 830. This should probably be handled by the NETCONF model and not the SSH server model.

[mattiaswal]

Blocked by #434, that includes the latest SSH model

[mattiaswal]

The netconf model just includes parts of the ssh model for SSH transport. @troglobit

[troglobit]

Looks like it might be better to just extend infix-services.yang.