installer: Require output dir specification
installer: Require output dir specification

Previously we'd hardcoded images/<partial checksum>, but I'm finding
that what I really want to do is stage my choice of images, and then
put them into place manually.  Or if not manually, in a more flexible
way that can be encoded into this script.

So this change then just forces callers to specify an output
directory.

projectatomic/rpm-ostree-toolbox,giuseppe/rpm-ostree-toolbox,markllama/rpm-ostree-toolbox,tlitovsk/rpm-ostree-toolbox,projectatomic/rpm-ostree-toolbox,giuseppe/rpm-ostree-toolbox,giuseppe/rpm-ostree-toolbox,sujitfulse/rpm-ostree-toolbox,sujitfulse/rpm-ostree-toolbox,projectatomic/rpm-ostree-toolbox,maxamillion/rpm-ostree-toolbox,giuseppe/rpm-ostree-toolbox,sujitfulse/rpm-ostree-toolbox,projectatomic/rpm-ostree-toolbox,sujitfulse/rpm-ostree-toolbox,giuseppe/rpm-ostree-toolbox,maxamillion/rpm-ostree-toolbox,tlitovsk/rpm-ostree-toolbox,tlitovsk/rpm-ostree-toolbox,tlitovsk/rpm-ostree-toolbox,maxamillion/rpm-ostree-toolbox,maxamillion/rpm-ostree-toolbox,markllama/rpm-ostree-toolbox,tlitovsk/rpm-ostree-toolbox,markllama/rpm-ostree-toolbox,maxamillion/rpm-ostree-toolbox,sujitfulse/rpm-ostree-toolbox,tlitovsk/rpm-ostree-toolbox,markllama/rpm-ostree-toolbox,markllama/rpm-ostree-toolbox,giuseppe/rpm-ostree-toolbox,projectatomic/rpm-ostree-toolbox,maxamillion/rpm-ostree-toolbox,sujitfulse/rpm-ostree-toolbox,markllama/rpm-ostree-toolbox