Fix REAMDE in contrib/terraform/aws template

[Brain2life]

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespaces from that line:

/kind api-change
/kind bug
/kind cleanup
/kind design

/kind documentation

/kind failing-test
/kind feature
/kind flake

What this PR does / why we need it:
The documentation in contrib/terraform/aws is outdated and misleading in some places. The main issue is that it mentions ubuntu and centos usernames, while in fact we are using admin user. I've lost some time to figure out why Ansible was not able to provision the cluster.

I've updated the documentation and replaced the usernames to admin. Please review and update the documentation.

Thank you!

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[linux-foundation-easycla]

• ❌ - login: @Brain2life / name: Maxat Akbanov . The commit (709f0b8, e2a7fdb, 4f81ac1) is not authorized under a signed CLA. Please click here to be authorized. For further assistance with EasyCLA, please submit a support request ticket.

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @Brain2life. Thanks for your PR.

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[yankay]

/ok-to-test