Allow server creation with no SSH key pair

I may be out to lunch on this, but [it looks like](https://github.com/browsh-org/baas/blob/8b4ffd37eabdc397bf76f710ad73cee3f211d508/ssh-server/ssh-server.go#L100) we currently require an SSH keypair to start our ssh-server. Guessing we mount the key into our container on startup? In Browsh's issue [#188](https://github.com/browsh-org/browsh/issues/188), it could help this user to simply be able to run the image with no keypair setup. Is this possible? It looks like gliderlabs SSH library supports it.

Start in ssh-server/ssh-server.go around line 100 and inspect how the SSH keypair is required when the server starts. Check the linked gliderlabs SSH library behavior for running without a keypair. Done means the image can start an ssh-server without keypair setup while preserving the existing keyed behavior.

Written by the indexing model from the issue text.