Add requirements for command classes
Cmmands -> Command
Now SpecInfra has Commands::Base class, so don't need to define a dummy class for testing
Move Serverspec::Commands::AIX to SpecInfra::Command::AIX
Copy all methods from Serverspec::Commands::Base
Move Command::Linux and Command::Debian from serverspec
Remove shellwords because it is required in base class
Move Command::Darwin from serverspec