downstream-setup: add tasks to disable and enable yum repos.

downstream-setup: add tasks to disable and enable yum repos.

This works similar to downloading yum_repos. You can specific a repo
file to disable using 'disable_yum_repos' and enable them by using
'enable_yum_repos'. If you use 'disable_yum_repos' with 'cleanup' it
will enable the repos listed in 'disable_yum_repos' - this feature is
used with teuthology and the Ansible task.

Signed-off-by: Andrew Schoen <aschoen@redhat.com>