A custom yum repository can now be configured by defining
`DIB_YUM_REPO_PACKAGE` as a yum available package or a URL to an rpm file.
This package can install repo files with any associated keys and
certificates.
A good example of such a package upstream is rdo-release[1] which
includes multiple repo files, the repo keys, and a root certificate.
This makes these repos impractical to install via DIB_YUM_REPO_CONF.
Downstream, repo packages like this a frequently used to bootstrap
development builds of RHEL with development repos.
[1] https://www.rdoproject.org/repos/rdo-release.rpm
Change-Id: I2832e723998c9bd7635cdf7541a4c20eff6294d2
