[docker][elastic-agent] use Default policy for enrollment

[simitt]

Related to changes from #24220. If an agent should be enrolled to the default policy, do not put the burden of knowing the default policy name on the user, but fall back to the default policy automatically.

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[elasticmachine]

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

[elasticmachine]

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• Build Cause: Pull request #24385 opened

• Start Time: 2021-03-05T13:45:17.285+0000

• Duration: 68 min 10 sec

• Commit: ab2edfc

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Passed	6548
Skipped	16
Total	6564

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Passed	6548
Skipped	16
Total	6564

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Pinging @elastic/agent (Team:Agent)

[blakerouse]

This is doing it the same way as bash was doing it. That is the wrong way to do this. Kibana marks the default policy with is_default. The code already uses that to find the correct policy.

See the findPolicy function in container.go.

[simitt]

I ran into a Error: unable to find policy named "" error without specifying the policy name. But this was due to Kibana not having the Fleet Server setup enabled by default; was merged with elastic/kibana#92422 and should be resolved with the next succesful snapshot build. Thanks.