OCM-6883 | fix: rosa describe admin can list the admin with custom name

[marcolan018]

https://issues.redhat.com/browse/OCM-6883

Signed-off-by: marcolan018 llan@redhat.com

In this PR, rosa describe admin will list out all users in cluster-admins group.
For the commands rosa create admin and rosa delete admin, they continue to only handle the cluster-admin user in cluster-admins group. Update help messages for those commands to make this more clear

[marcolan018]

/retest

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 21.49%. Comparing base (94a5865) to head (d87f940).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1873      +/-   ##
==========================================
+ Coverage   21.47%   21.49%   +0.01%     
==========================================
  Files         113      113              
  Lines       18664    18668       +4     
==========================================
+ Hits         4009     4013       +4     
  Misses      14349    14349              
  Partials      306      306              

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[ciaranRoche]

/lgtm

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ciaranRoche, marcolan018

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