Add SSH access method

Add SSH access to the BigBlueButton servers. In this issue, we first want to add a general method that allows sending commands to the server and retrieves the result.

1. A method on the BigBlueButton class. It must take the command, arguments, and stdin, and return the exit code, stdout, and stderr.
2. A method on the BigBlueButtonGroup class that calls the method on all member servers
3. A command group system in bbb-cli with a subcommand sshthat takes an arbitrary command, executes it on the group, and prints the output from each server in order. It should also multiplex stdin and pass it to each server when calling the command.