Fixes Bug: "yum repo don't work in CentOS 6.4"

Removing curly braces around the the basearch parameter which
lead to parsing issues.

Fixing additional issues with yum repo configuration and
adapting the CI test.

Locally verified this with RS_SET=two-centos-64-x64.
While it would be preferable to verify it via gerritexec,
all of the CI except ceph_repo_spec.pp fail.
This will be handled with subsquent patches.

Change-Id: Ia99e20dab8c496cc3becec77acbe38003c6d7779
Closes-Bug: 1264959
Partially-Bug: 1264916