publish pulpcore and pulp_rpm to PyPI pre-releases nightly

[daviddavis]

Migrated from https://pulp.plan.io/issues/7608

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Nightly integration testing with Katello requires Pulp publishing nightly pre-releases. Plugin template needs to provide an optional stage for such deployment. This stage needs to also clean up any pre-release builds that are older than 30 days.

[stale]

This issue has been marked 'stale' due to lack of recent activity. If there is no further activity, the issue will be closed in another 30 days. Thank you for your contribution!