Updating procedure to improve image-builder user mangement

[jaradtke-aws]

Issue #, if available: The procedure for managing the image-builder user in the Operating Systems Management | Artifacts page contained unnecessary steps that modified the underlying OS. The goal is to add a user (image-builder) and configure the system to allow that user to run the image-builder process. That requires that the user can access /dev/kvm (which has permissions: crw-rw---- root:kvm) indicating that the user needs to either be the root user, or belong to the kvm group. This PR was created for Issue #7846 https://github.com/aws/eks-anywhere/issues/7846

Description of changes: Update the user management procedure to be OS-specific. Added additional configuraiton to add the image-builder user to the appropriate groups at creation time (sudo/wheel,kvm). replaced "/home/$USER" with "${HOME}" throughout entire document. removed suggestions to modify OS file permissions to /dev/kvm.

NOTE: there are essentially 2 procedure for creating an image (requires KVM: Bare Metal/Cloud Stack - the other methods do not)

Removed any steps demonstrating how to modify the OS

-   sudo usermod -a -G kvm $USER
-   sudo chmod 666 /dev/kvm
-   sudo chown root:kvm /dev/kvm

Updated the step to create a user to have that step include the kvm and "admin" group at that time (several examples provided here)

-   sudo adduser image-builder
+   sudo useradd -G kvm,sudo image-builder
+   sudo useradd -G kvm,wheel image-builder
+   sudo useradd -G sudo image-builder
+   sudo useradd -G wheel image-builder

If the user was created using the updated useradd command, then the usermod step is unnecessary. Therefore removed that step

-   sudo usermod -aG sudo image-builder

Testing (if applicable): The following was run on an Ubuntu 22.04

# sudo useradd -G kvm,sudo testuser
$ groups testuser
testuser : testuser kvm
$ grep '^NAME|^VERSION' /etc/*release*
/etc/os-release:NAME="Ubuntu"
/etc/os-release:VERSION_ID="22.04"
/etc/os-release:VERSION="22.04.4 LTS (Jammy Jellyfish)"
/etc/os-release:VERSION_CODENAME=jammy

The following was run on Red Hat Enterprise Linux 8

$ sudo useradd -G kvm,wheel testuser
$ groups testuser
testuser : testuser wheel kvm
$ egrep '^NAME|^VERSION' /etc/*release*
/etc/os-release:NAME="Red Hat Enterprise Linux"
/etc/os-release:VERSION="8.9 (Ootpa)"
/etc/os-release:VERSION_ID="8.9"

Documentation added/planned (if applicable):

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 73.39%. Comparing base (0e02f8e) to head (00cd95b).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7852   +/-   ##
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  Coverage   73.39%   73.39%           
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  Files         576      576           
  Lines       35622    35622           
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  Hits        26144    26144           
  Misses       7826     7826           
  Partials     1652     1652           

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