Fix Agent failing to run in standalone mode

[aleksmaus]

What does this PR do?

Fixes Agent failing to run in standalone mode.

Error: could not load agent info: storing generated agent id: open /usr/share/elastic-agent/state/data/vault/2bb80d5...7bf527a25b: no such file or directory

Related to the following issue: elastic/cloud-on-k8s#5720

The root cause of the issue here is that Agent when running standalone tries to write the agentID into fleet.enc (which is encrypted former fleet.yml).
If fails because the the encryption seed was not created if the agent was not enrolled.

The fix here is to create the agent secret on run. This will not create the new key if there is one that already exists after enrollment. At it allows the agent to write the agentID into fleet.enc file.

Why is it important?

Fixes Agent failing to run in standalone mode: elastic/cloud-on-k8s#5720

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas

How to test this PR locally

Run the agent locally without enrolling it first with fleet.

Related issues

• Closes TestFleetMode/ES_data_should_pass_validations is flaky (again) with 8.3.0-SNAPSHOT cloud-on-k8s#5720

Screenshots

The agent runs in the standalone mode successfully.

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• Start Time: 2022-06-02T16:19:54.476+0000

• Duration: 38 min 58 sec

Test stats 🧪

Test	Results
Failed	0
Passed	5973
Skipped	23
Total	5996

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Name	Metrics % (covered/total)	Diff
Packages	97.183% (69/71)	👍
Files	68.493% (150/219)	👍
Classes	68.109% (299/439)	👍
Methods	51.7% (821/1588)	👍
Lines	38.064% (8806/23135)	👍 0.011
Conditionals	100.0% (0/0)	💚