Fully support the --dir option

We currently don't support the --dir flag to openshift-install. Previously this was partly due to not loading files as assets. Now the principal remaining culprit is outputting the agent ISO, which is too large to be returned from Files() (which contains a []byte) so that the entire contents are stored in memory, and also written to the asset store.

A couple of proposals for fixes exist:

• https://github.com/openshift/installer/pull/5990
• https://github.com/openshift/installer/pull/6009

Once this is done, we can test it by cleaning up to dev-scripts code to use the flag instead of changing the working directory: https://github.com/openshift-metal3/dev-scripts/blame/master/agent/05_agent_create_cluster.sh#L19-L23