[OSASINFRA-IPV6 DT]Set MTU to 9000

[rlobillo]

With 1500 MTU, we hit https://access.redhat.com/solutions/7088208

Same is not hit in ipv4 because the overhead is higher in ipv6. This patch is updating the MTU to 9000 so the shiftstack installation on top of this DT can succeed.

[openshift-ci]

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[abays]

@rlobillo: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:
Test name Commit Details Required Rerun command
ci/prow/images 7f474db link true /test images

Full PR test history. Your PR dashboard.

{  error occurred handling build src-amd64: could not get build src-amd64: builds.build.openshift.io "src-amd64" not found}

/test images

[rlobillo]

/hold

I'm confirming that this approach is necessary.

[openshift-ci]

@rlobillo: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
ci/prow/images	7f474db	link	true	/test images

Full PR test history. Your PR dashboard.

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