As noted by lbragstad, we need to make ImageTarget contain a member
field so that we can generically apply policies and be able to
properly include images for which we are a member. This was hacked
into place for ImageRepoProxy.get() but in order to apply it generally
to listing and other ops, we need to formalize that.
Partially-Implements: blueprint policy-refactor
Change-Id: I92d3792602a69922078d109095ad8ac9afc89d14