* I had to contextualise Communicator.Start and RemoteCmd.StartWithUi
NOTE: Communicator.Start starts a RemoteCmd but RemoteCmd.StartWithUi will run the cmd and wait for a return, so I renamed StartWithUi to RunWithUi so that the intent is clearer.
Ideally in the future RunWithUi will be named back to StartWithUi and the exit status or wait funcs of the command will allow to wait for a return. If you do so please read carrefully https://golang.org/pkg/os/exec/#Cmd.Stdout to avoid a deadlock
* cmd.ExitStatus to cmd.ExitStatus() is now blocking to avoid race conditions
* also had to simplify StartWithUi
This provides an alternate mode for the amazon-chroot builder which uses
a blank volume to build the image. It adds StepPreMountCommands to
permit partitioning and format commands to be executed before mounting
the new volume.
Adrien Delorme
Megan Marsh
Matthew Hooker
Jeremy Asher