check domain join based on the gMSA support env

[saikiranakula-amzn]

Summary

Fix to validate the domain join only if gMSA support is enabled

fixes error log messages on non-gMSA instances (ie, most instances) like this:

level=error time=2022-XX-XXT03:12:09Z msg=""CREDENTIALS_FETCHER_HOST_DIR not found, err: stat /var/credentials-fetcher/socket/credentials_fetcher.sock: no such file or directory"" module=parse_linux.go"
level=error time=2022-XX-XXT03:12:09Z msg=""CREDENTIALS_FETCHER_HOST_DIR not found, err: stat /var/credentials-fetcher/socket/credentials_fetcher.sock: no such file or directory"" module=parse_linux.go"

Implementation details

Testing

New tests cover the changes:

Description for the changelog

bugfix: don't log errors on instances not using GMSA

Licensing

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.