Reset yum configuration in post-install.d

When using the yum element, we should reset the changes we've made to
/etc/yum.conf during post-install.d. Otherwise, this build time
configuration is propagated into booted instances.

Change-Id: I1eea586ca0fefe9bc0cf91fedefcbd141a536fa2
Merge "Reset yum configuration in post-install.d"