fix: build each repository in the right branch

[mdelapenya]

What does this PR do?

This PR builds the affected repos in the right branches:

• fleet-server: if there is a payload comming from a PR in the repo (via GH comment), it will build the 'PR/#ID' branch. Same if it's manually triggered using a PR ID as input paramenter. If a branch is used as input parameter (i.e. master, 7.16, 7.x) the the branch will be used as is.
• beats: it will use what comes in the $BASE_REF variable, which is set in the first stage of the pipeline, within the getE2EBaseBranch() helper method. This method will internally set the env var to the baseRef attribute of the payload retrieved by our API call, which uses what comes from the webhook or in the input parameter. If a branch is used as an input parameter, it will build the same Beats branch.

Why is it important?

Build what is right

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 (make unit-test), 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)

How to test this PR

Manually trigger the https://beats-ci.elastic.co/job/e2e-tests/job/e2e-testing-fleet-server/ job, using a fleet-server PR (or a branch) as parameter.

Related issues

• Continues feat: run E2E tests for Fleet Server #1628

[mdelapenya]

Merging, as it's not affecting the test suite

[elasticmachine]

💔 Build Failed

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Build stats

• Start Time: 2021-10-01T14:04:12.506+0000

• Duration: 2 min 24 sec

• Commit: 777108a

Steps errors

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Git fetch

• Took 0 min 1 sec . View more details here
• Description: git fetch https://${GIT_USERNAME}:${GIT_PASSWORD}@github.com/elastic/e2e-testing.git +refs/pull/*/head:refs/remotes/origin/pr/* > fetch.log 2>&1

Archive the artifacts

• Took 0 min 1 sec . View more details here
• Description: fetch.log

❕ Flaky test report

No test was executed to be analysed.

🤖 GitHub comments

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