Reference /etc/gitlab/initial_root_password for default password

What does this MR do?

The Amazon POC assumes that you are using the official GitLab AMI to build the GitLab environment https://docs.gitlab.com/ee/install/aws/manual_install_aws.html#install-gitlab-and-create-custom-ami

The same section also talks about manually installing GitLab:

If you prefer, instead of using the official GitLab AMI, you can also spin up an EC2 instance of your choosing and manually install GitLab.

The problem is that our documentation does not tell the user where to find the initial root password if the user decides to install GitLab manually and they choose not to use the official GitLab AMI. In this situation, using the instance ID for the password will not work as that is specific to the GitLab AMI.

Updated the docs to point to the location where they can find the initial root password.

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