Add option to skip auto-creating ssh_endpoint

The current behaviour is to create an SSH endpoint on port 22 (if
available), or on a random port if it's not, for any new Linux VM.

This commit allows this endpoint creation to be skipped, which is
useful for machines on a virtual network, which don't require
public remote access.

To skip the endpoint creation, simply add no_ssh_endpoint: true 
to the options hash for a new deployment.