Conformance tweaks for Linkerd and other meshes

[kflynn]

/kind bug
/area conformance

This PR makes some small changes to allow conformance to work for Linkerd and, most likely, other meshes that don't also provide gateway controllers. Huge props to @adleong for doing most of the heavy lifting on this, weeks ago. 🙂

Like most nontrivial PRs of mine, it is best reviewed commit by commit.

• Allow -namespace-annotations to annotate namespaces created by the tests

  We already had -namespace-labels to supply labels on test namespaces. Linkerd needs annotations rather than labels, so we add a new parallel flag to set annotations on test namespaces.

• Document -namespace-annotations

  Documentation is in a separate commit.

• comment fix

  This just triggered me so I had to fix it. 🙂

• Split out MeshNamespacesMustBeReady, since mesh namespaces don't have Gateways

  If testing a service mesh that doesn't also provide a gateway controller, there won't be any Gateways in the namespace, so this pulls out a separate function to check that the namespace is ready without needing Gateways, and uses it for the namespaces used for mesh testing.

• MeshFrontendHostname requires HTTPResponseHeaderModification

  The MeshFrontendHostname test actually relies on a header modification, but wasn't tagged as such.

Does this PR introduce a user-facing change?:

Better support mesh-only conformance testing

[keithmattix]

Looks good; will wait for a bit for john to have a look

[robscott]

Thanks @kflynn!

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: howardjohn, keithmattix, kflynn, robscott

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