warn about joining controllers in parallel

[ajp-io]

What this PR does / why we need it:

Warn that you shouldn't join another node when a controller is being joined.

Also update other warns to use logrus.Warn.

Which issue(s) this PR fixes:

https://app.shortcut.com/replicated/story/106755/warn-that-you-should-only-join-one-node-at-a-time

Does this PR require a test?

NONE

Does this PR require a release note?

When joining a controller node, warn that the user shouldn't join another node until the controller joins successfully.

Does this PR require documentation?

replicatedhq/replicated-docs#3126

[github-actions]

This PR has been released (on staging) and is available for download with a embedded-cluster-smoke-test-staging-app license ID.

Online Installer:

curl "https://staging.replicated.app/embedded/embedded-cluster-smoke-test-staging-app/ci/appver-dev-e3ed978" -H "Authorization: $EC_SMOKE_TEST_LICENSE_ID" -o embedded-cluster-smoke-test-staging-app-ci.tgz

Airgap Installer (may take a few minutes before the airgap bundle is built):

curl "https://staging.replicated.app/embedded/embedded-cluster-smoke-test-staging-app/ci-airgap/appver-dev-e3ed978?airgap=true" -H "Authorization: $EC_SMOKE_TEST_LICENSE_ID" -o embedded-cluster-smoke-test-staging-app-ci.tgz

Happy debugging!

[emosbaugh]

Failures are unrelated to changes. Merging