Support installing packages in IPA images

Diskimage builder supports specifying a list of packages to install via
-p. We currently support this for the root image, but not IPA images.

This change adds a new configuration option, 'ipa_build_dib_packages',
that should be a list of additional packages to install in locally built
IPA images. This affects the following commands:

kayobe seed deployment image build
kayobe overcloud deployment image build

This depends on
https://github.com/stackhpc/ansible-role-os-images/pull/25.

Also adds an example to the documentation for the equivalent option for
root image.

Change-Id: I04191d9541894b6a264e966c9ecb1056e0edade4
Story: 2006855
Task: 37446