Update the maas build scripts to use the ppc64el config on stilson which reports `ppc64le` for `uname -m` and use a raw disk attached to /dev/vdc.
[a=fginther][r=philroche,rcj]
MP: https://code.launchpad.net/~ubuntu-on-ec2/vmbuilder/jenkins_kvm-maas-stilson2/+merge/348867