Add changelog note for breaking change in #371

[ehashman]

What this PR does / why we need it:

Adds a note about breaking changes to the CHANGELOG for the 1.2.0 -> 1.3.0 upgrade.

The changelog does not currently warn about the deprecation of the --in-cluster flag, meaning that users on 1.2.0 releases and earlier who use --apiserver with the default --in-cluster=true flag have authentication break on upgrade.

Which issue(s) this PR fixes:

Fixes #543.

Other notes:

The behaviour from 1.2.0 can be emulated on upgrade by removing the --apiserver flag and setting the environment variable KUBERNETES_SERVICE_HOST to the API server value.

[mxinden]

Would you mind mentioning the fix via KUBERNETES_SERVICE_HOST in the changelog as well? Other than that this looks good to me, thanks for the follow up!

[andyxning]

Please also add a notation about the deprecation PR #371

[ehashman]

All feedback should be addressed.

[andyxning]

@ehashman You should add a link to the #371 PR to make it a link instead of just a statement.

[ehashman]

I figured it would auto-link but apparently not...

[andyxning]

/lgtm
/approve

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: andyxning, ehashman

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[andyxning]

@ehashman Thanks for taking care of this!