DIB does not work for RHEL7 on ppc64le

[Sudipta Biswas]

Currently, the diskimage-builder does not work for RHEL7 on ppc64le.
I could identify two places where the changes would be needed but there are more places
that will have to be fixed in order to support this.

The following are the changes I have figured out:

1. https://github.com/openstack/diskimage-builder/blob/14152b1657a87afdfb60f5ae978db174ae599693/elements/rhel7/root.d/10-rhel7-cloud-image#L12

Here the supported arch needs to include ppc64le
2. https://github.com/openstack/diskimage-builder/blob/14152b1657a87afdfb60f5ae978db174ae599693/elements/rpm-distro/pre-install.d/01-override-yum-arch#L18

Here another check for ppc64le as arch needs to be put.

I would like to use this bug to fix all other issues that may arise during the actual build of the RHEL7 ppc64le image.

[Chhavi Agarwal (chhagarw)]

Another issue which I hit while running for ppc64el, the below repos does not exist which are enabled by default in DIB
repos="repos --enable rhel-7-server-rpms"
satellite_repo="rhel-7-server-rh-common-rpms"