Cherry-pick #21951 to 7.x: Always try snapshot repo for agent upgrade

[michalpristas]

Cherry-pick of PR #21951 to 7.x branch. Original message:

What does this PR do?

This PR adds ability of agent to download new version from snapshot repo. This makes update from stable version e.g 7.10 to 8.0.0 SNAPSHOT possible.

Why is it important?

For this to be possible user needs to initiate upgrade using API and provide sourceURI or Kibana needs to be upgraded from stable to SNAPSHOT before agent, so this is not a very common scenario but we're not blocking it.

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 added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

cc @EricDavisX

[elasticmachine]

Pinging @elastic/ingest-management (Team:Ingest Management)

[elasticmachine]

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Build stats

• Build Cause: [Pull request #22010 opened]

• Start Time: 2020-10-20T09:39:09.612+0000

• Duration: 38 min 10 sec

Test stats 🧪

Test	Results
Failed	0
Passed	1394
Skipped	4
Total	1398

[blakerouse]

Looks good