Fix bad yum cache during repo update

When deploying rocky, python2-tripleo-repos fails to install because of
a bad yum cache even after updating the repo. This patch just clears the
yum cache after updating the repo.

Change-Id: I94a68611faf1c3ff89d57c3d877d3336574ce8bb
Signed-off-by: Tim Rozet <trozet@redhat.com>