Fedora: Add support for configuring Yum mirrors

Updates the fedora element so that DIB_DISTRIBUTION_MIRROR can
be used to configure a custom Yum mirror on Fedora. This
implementation replaces the baseurl in each of the default
Fedora Yum .repo files so that a custom mirror URL/path
can be supplied. The URL should point to the directory
where the updates and release directories exist. Example:

  http://download.fedoraproject.org/pub/fedora/linux

As part of this change we also remove any existing
metalink settings in the .repo files to cover the case
where an end user runs their own createrepo command
to create the mirror... which means your metadata won't match that
of the normal Fedora mirrors and your baseurl won't get used.

Change-Id: I1a49a2941b1cc489e91a72b87430393cd32e865d
Merge "Fedora: Add support for configuring Yum mirrors"