Add element to remove serial console.

Upstream cloud images likely have console=ttyS0 embedded in the kernel
commandline portion of the GRUB config. This makes local, direct testing
of the images substantially more difficult because upstart is writing
its output to a console that isn't being monitored.
This element simply removes console=ttyS0 from the grub configuration.

Change-Id: I07429bb504b467634071cc31ef184af730bbb5d6
Merge "Add element to remove serial console."