Add "join" command to form a complete cluster

Add join command

The join command installs the agent software on a remote host
after having fetched the node-token from a k3s server specified
via the --server-ip flag.

Tested with 2x Ubuntu 18.04 machines on Civo Cloud using the
root user for ssh.

Closes: #3

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <alexellis2@gmail.com>

Initial working release

Split out ssh package

Signed-off-by: Alex Ellis <alexellis2@gmail.com>