add option to skip the confirmation dialog on start

[ThinkOpenly]

Currently, mambo will always pause with the dialog:

----------------------------------------------

You are starting the IBM POWER9 Functional Simulator
When the boot process is complete, use the following
credentials to access it via ssh:

     ssh root@172.19.98.109
     password: mambo

     Would you like to start it now? [Y/n]

It would be nice for those that frequently start an instance of the simulator, or to easily allow automatic start-up, to add an option to skip this dialog as if the user hit "Y". Example:

$ mambo --start power9 --yes