fix: enhance GCP queries for PRs

[mdelapenya]

What does this PR do?

It enriches the logs when querying the GCP storage bucket, also reducing the amount of entries to look up adding a filter in the form of a prefix=PREFIX query string parameter to limit results to objects that have the specific prefix (See https://cloud.google.com/storage/docs/listing-objects#rest-list-objects).

As a consequence, we noticed that the filename used for the TAR.GZ files for snapshots was incorrect, as it needs the OS after version and before architecture.

Why is it important?

Beats packaging jobs were using wrong artifacts, because of several reasons:

1. Beats packaging job was setting the wrong parameter when triggering this job. It caused that the snapshot in the GCP bucket was never used, using the released artifacts from artifactory.
2. Once fixed, the file name for TAR.GZ artifacts was not built properly.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have run the Unit tests for the CLI, and they are passing locally
• I have run the End-2-End tests for the suite I'm working on, and they are passing locally
• I have noticed new Go dependencies (run make notice in the proper directory)

Author's Checklist

• We follow GCP docs for listing objects: https://cloud.google.com/storage/docs/listing-objects#rest-list-objects
• File names for TAR.GZ and DEB/RPM differ, as the latters do not add the O.S.

How to test this PR locally

SUITE="fleet" \
    TAGS="fleet_mode && enroll" DEVELOPER_MODE=true \
    TIMEOUT_FACTOR=3 LOG_LEVEL=TRACE \
    ELASTIC_AGENT_VERSION="pr-22790" ELASTIC_AGENT_USE_CI_SNAPSHOTS=true \
    make -C e2e functional-test

Related issues

• Closes CI Snapshots is not finding pull request objects because of the huge amount of objects #503
• Relates fix: use proper param name for e2e tests beats#22836

Use cases

• Beats PRs triggering the packaging job will trigger the E2E tests. They need to run the tests using the snapshots produced by the CI
• Local execution of the tests reproducing CI steps, which is useful for manual testing already pushed artifacts/snapshots.

[elasticmachine]

💚 Build Succeeded

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Build stats

• Build Cause: Started by user Manuel de la Peña

• Start Time: 2020-12-02T06:15:14.585+0000

• Duration: 29 min 44 sec

Test stats 🧪

Test	Results
Failed	0
Passed	45
Skipped	0
Total	45

💚 Flaky test report

Tests succeeded.

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Test stats 🧪

Test	Results
Failed	0
Passed	45
Skipped	0
Total	45

[mdelapenya]

I verified that running this PR against an existing snapshot (elastic/beats#22790) uses it during tests: https://beats-ci.elastic.co/job/e2e-tests/job/e2e-testing-mbp/job/PR-504/3/parameters/