Allow shutit to connect over ssh

````
./shutit build -m library -s shutit.tk.casperjs.casperjs build yes -s build conn_module shutit.tk.conn_ssh -s shutit.tk.conn_ssh ssh_host 172.17.0.9 -s shutit.tk.conn_ssh ssh_user myuser -s shutit.tk.conn_ssh ssh_key ~/.ssh/id_rsa
````
Full list of options from util.py:

````
[shutit.tk.conn_ssh]                                                            
# Required                                                                      
ssh_host:                                                                       
# All other configs are optional                                                
ssh_port:                                                                       
ssh_user:                                                                       
password:                                                                       
ssh_key:                                                                        
# (what to execute on the target to get a root shell)                           
ssh_cmd:
````

There's probably a bunch of stuff that assumes the presence of docker (e.g. checking cidfiles etc), but this at least seems to minimally work.
Ideally now all the docker specific stuff from [build] (e.g. lxc_conf) would now be moved under shutit.tk.conn_docker.