Create asset graph engine

[yifan-gu]

This PR establishes the initial framework for generating assets using a graph engine (https://jira.coreos.com/browse/CORS-758).

• Adds an openshift-install binary for generating assets. This is not hooked up to the build process yet. To build the executable, run go build ./cmd/openshift-install.
• Adds an asset.Store type that generates an asset by walking the tree of dependencies for the asset ensuring that each asset of a dependency has been generated prior to generating the dependent asset.
• Adds an asset.Stock type that establishes the available assets that can be generated.
• Adds the install-config asset and its dependent assets (https://jira.coreos.com/browse/CORS-760, https://jira.coreos.com/browse/CORS-759). To generate the install-config asset, run ./openshift-install install-config. The user-provided assets as crude with respect to UX and still need further refinement. The install-config asset does not fully populate the install-config.yml yet.
• Vendors https://github.com/stretchr/testify to provide some convenience functions for use in unit tests.
• Vendors https://github.com/pborman/uuid to generate a random UUID for the cluster ID.

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[yifan-gu]

Take from #120, and rebased on master, to see if it can pass the tests

[openshift-ci-robot]

@yifan-gu: The following test failed, say /retest to rerun them all:

Test name	Commit	Details	Rerun command
ci/prow/e2e-aws	8fc4566	link	/test e2e-aws

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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[yifan-gu]

Looks like the same thing, so closing and wait for @crawford to have some fix up.