Container CI testing shouldn't install packages onto the pulp-api container

[fao89]

Author: @mikedep333 (mdepaulo@redhat.com)

Redmine Issue: 5404, https://pulp.plan.io/issues/5404

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Currently in order to do CI testing, plugin-template installs RPM and python packages into the pulp-api container.

This has 3 problems:
1. It is brittle. If pulp-api is restarted, they are lost.
2. It means the container image is being modified; which could possibly invalidate tests.
3. It is probably not the correct design pattern.

This task is to reach the correct design pattern, and implement it.
A possible alternative pattern is running tests from a separate sidecar container in the same pod.

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