[Fleet] Added missing option to fleet server intructions

[juliaElastic]

Summary

Fixes #119606

To verify:

1. Go to Fleet
2. In Fleet Server instructions, select Quick start, click through to see command
3. Verify that command contains --fleet-server-insecure-http
4. Switch to production mode, verify that the argument in 3. is not there
5. Copy the command from quick start and verify that fleet server is successfully enrolled with it

juliabardi@Julias-MacBook-Pro elastic-agent-7.16.0-darwin-x86_64 % sudo ./elastic-agent install  \
  --fleet-server-es=http://localhost:9200 \
  --fleet-server-service-token=AAEAAWVsYXN0aWMvZmxlZXQtc2VydmVyL3Rva2VuLTE2Mzk0ODgxMTE5MTE6NUZLb2dtcDRSNldaZHBrZ1g4SGdUdw \
  --fleet-server-policy=499b5aa7-d214-5b5d-838b-3cd76469844e \
  --fleet-server-insecure-http
Elastic Agent will be installed at /Library/Elastic/Agent and will run as a service. Do you want to continue? [Y/n]:Y
2021-12-14T14:23:35.226+0100	INFO	cmd/enroll_cmd.go:727	Waiting for Elastic Agent to start Fleet Server
2021-12-14T14:23:37.228+0100	INFO	cmd/enroll_cmd.go:760	Fleet Server - Starting
2021-12-14T14:23:49.230+0100	INFO	cmd/enroll_cmd.go:741	Fleet Server - Running on policy with Fleet Server integration: 499b5aa7-d214-5b5d-838b-3cd76469844e; missing config fleet.agent.id (expected during bootstrap process)
2021-12-14T14:23:49.230+0100	WARN	[tls]	tlscommon/tls_config.go:98	SSL/TLS verifications disabled.
2021-12-14T14:23:50.009+0100	INFO	cmd/enroll_cmd.go:442	Starting enrollment to URL: http://localhost:8220/
2021-12-14T14:23:52.919+0100	INFO	cmd/enroll_cmd.go:250	Successfully triggered restart on running Elastic Agent.
Successfully enrolled the Elastic Agent.
Elastic Agent has been successfully installed.

Prod options unchanged:

Checklist

• Unit or functional tests were updated or added to match the most common scenarios

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[nchaulet]

LGTM 🚀

[kibana-ci]

💚 Build Succeeded

• Buildkite Build
• Commit: 4933de7
• Storybooks Preview

Metrics [docs]

Async chunks

Total size of all lazy-loaded chunks that will be downloaded as the user navigates the app

id	before	after	diff
fleet	641.9KB	641.9KB	+48.0B

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

cc @juliaElastic

[kibanamachine]

💔 Backport failed

Status	Branch	Result
✅	8.0
❌	7.16	Commit could not be cherrypicked due to conflicts

Successful backport PRs will be merged automatically after passing CI.

To backport manually run:
node scripts/backport --pr 121180