CI: Remove IPA images from saved /etc/kolla

This saves 156MB of storage in each CentOS job on the master branch.

Backport note: ironic-ipxe is Xena only. Storage space saved on Wallaby
and older will be about half (varies depending on Linux distribution and
OpenStack release).

Change-Id: Icb3569bc764435127a00ee15583c6f3e4e598eca