Allow role_arn work with access keys for AWS

[kaiyan-sheng]

What does this PR do?

This PR is to change role_arn config option to work not only with given credential profile but also access keys. With this change, aws integration will be able to use role_arn without volume mounting the credential profile file.

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.

How to test this PR locally

With IAM role test-role setup, config below should work:

metricbeat.modules:
- module: aws
  period: 5m
  access_key_id: xxx
  secret_access_key: yyy
  role_arn: arn:aws:iam::1234:role/test-role
  metricsets:
    - ec2

Related issues

This PR also fixed the role_arn related documentation.

• Closes [DOCS] Clarify aws credentials required with role_arn #24911

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Build stats

• Build Cause: Pull request #25446 updated

• Start Time: 2021-05-03T13:44:18.668+0000

• Duration: 152 min 3 sec

• Commit: f148d38

Test stats 🧪

Test	Results
Failed	0
Passed	18975
Skipped	1577
Total	20552

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Test stats 🧪

Test	Results
Failed	0
Passed	18975
Skipped	1577
Total	20552