fix: update flags for the enroll command

[mdelapenya]

What does this PR do?

This PR appends the --url= flag to the Kibana URL and the --enrollment-token= flag to the API token arguments when enrolling an agent using Fleet mode.

As it's used in three different places, we are extracting the array representing the arguments to a builder method (it requires a token), so that further updates will affect only one place.

Why is it important?

The enrollment command changed in elastic/beats#24155

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 run the Unit tests for the CLI, and they are passing locally
• I have run the End-2-End tests for the suite I'm working on, and they are passing locally
• I have noticed new Go dependencies (run make notice in the proper directory)

Author's checklist

• @michalpristas does the install command call need to be updated?
• For install command, Kibana flag uses --kibana-url, while for the enroll command uses --url. Is this inconsistency on purpose?

How to test this PR locally

SUITE="fleet" TAGS="fleet_mode_agent && install" TIMEOUT_FACTOR=3 LOG_LEVEL=TRACE DEVELOPER_MODE=true ELASTIC_APM_ACTIVE=false make -C e2e functional-test

Related issues

• Caused by [Elastic Agent] Fix docker entrypoint for elastic-agent. beats#24155
• Fixes Update enrollment command  #803

[michalpristas]

seems ok

[elasticmachine]

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• Build Cause: mdelapenya commented: jenkins run the tests please

• Start Time: 2021-02-23T14:08:01.845+0000

• Duration: 24 min 59 sec

• Commit: a2e3b0c

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Test	Results
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Passed	117
Skipped	0
Total	117

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Passed	117
Skipped	0
Total	117

[mdelapenya]

jenkins run the tests please

[mdelapenya]

I experimented some inconsistencies in the artifactory: https://artifacts-api.elastic.co/v1/search/8.0.0-SNAPSHOT/elastic-agent?x-elastic-no-kpi=true was not retrieving artifacts at the moment the CI requested the resource. Now it's available again