Cherry-pick #24128 to 7.x: Fix: Successfully installed and enrolled agent running standalone

[michalpristas]

Cherry-pick of PR #24128 to 7.x branch. Original message:

What does this PR do?

In #23865 there was a change in order of starting a service and enrollment itself.
Installation procedure first enrolled agent and then started a service.

With the change introduced we first start a service in standalone mode and then enroll to fleet/fleet-server if needed.
This lead into race which either did not manifest or manifested as agent with fleet config in it's directory running standalone

During enrollment there was a check if it originates in install command and if so restart is not performed. This check is no longer needed and restart needs to be performed every time.

Manifested as what we thought of as flaky-ness in e2e

Why is it important?

Successfully installed and enrolled agent running standalone

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.

[elasticmachine]

Pinging @elastic/agent (Team:Agent)

[elasticmachine]

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

[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 #24131 opened

• Start Time: 2021-02-19T15:50:46.016+0000

• Duration: 52 min 39 sec

• Commit: d441d3f

Test stats 🧪

Test	Results
Failed	0
Passed	6486
Skipped	24
Total	6510

Trends 🧪

💚 Flaky test report

Tests succeeded.

Expand to view the summary

Test stats 🧪

Test	Results
Failed	0
Passed	6486
Skipped	24
Total	6510