Add support for SSH publickey authentication

[lamhoangtung]

Hi, thanks for the great tools.

Since I hate to copy paste password to login to the server and also hate to mange many SSH key. I usually use my default SSH key to authenticate. That's why submitted this PR so everyone like me can just authenticate instantly without any hassle.

Hope you could merge this :P. Thanks a lot again

[demotomohiro]

Public key authentication is allowed in default and the default value of AuthorizedKeysFile is ".ssh/authorized_keys".
You can use public key authentication by adding code that save your public key to .ssh/authorized_keys.

For example:

!pip install git+https://github.com/demotomohiro/remocolab.git
import remocolab
remocolab.setupSSHD()

!mkdir /home/colab/.ssh
with open("/home/colab/.ssh/authorized_keys", 'w') as f:
  f.write("Content of my public key")
!chown colab /home/colab/.ssh /home/colab/.ssh/authorized_keys
!chmod 700 /home/colab/.ssh
!chmod 600 /home/colab/.ssh/authorized_keys