chore: initialise stale agent only when used

[mdelapenya]

What does this PR do?

It removes the initialisation of the stale agent in the fleet test suite, as it's already checked in the specific step for the stale agent. We delayed fetching the stale version in #1016

Why is it important?

It fails fast when the version is not present, and for the stale agent, which is only executed on nightlies, we don't want to fail fast.

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)

Related issues

• Caused by v2 refactor #1008
• Relates fix: delay checking stale agent version until it's used #1016

[elasticmachine]

💚 Build Succeeded

the below badges are clickable and redirect to their specific view in the CI or DOCS

Expand to view the summary

Build stats

• Build Cause: Pull request #1084 opened

• Start Time: 2021-04-23T19:26:09.785+0000

• Duration: 21 min 7 sec

• Commit: 2e37fde

Test stats 🧪

Test	Results
Failed	0
Passed	142
Skipped	0
Total	142

Trends 🧪

💚 Flaky test report

Tests succeeded.

Expand to view the summary

Test stats 🧪

Test	Results
Failed	0
Passed	142
Skipped	0
Total	142

[mdelapenya]

CI passing, merging to fix builds