Support delegate Contour IngressRoutes

[jonasrmichel]

This PR addresses #1143 by properly checking if an IngressRoute's Spec.VirtualHost field is defined (non-nil) during endpoint computation.

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[jonasrmichel]

/assign @Raffo

[jonasrmichel]

@Raffo and @njuettner -- Would either of you have a moment to give this PR a quick sanity check? It's a very minor fix to the Heptio Contour source implementation.

[njuettner]

/lgtm
/approve

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