Fix duplicate entry of admin user in the tacacs config

[veronica-arista]

Description of PR

PR11614 adds two new sets of credentials to the tacplus authentication config, "duthost_admin_user" and "ansible_ssh_user", however in our testbed set-up, these are both the DUT admin user, causing problems due to duplicate tacplus config. The observed behavior is as described in https://github.com/sonic-net/sonic-mgmt/issues/13708

This PR adds a check for whether ansible_ssh_user is the same as duthost_admin_user, preventing duplicate config.

Summary: Fixes #13708

Type of change

• [x] Bug fix
• [ ] Testbed and Framework(new/improvement)
• [ ] Test case(new/improvement)

Back port request

• [ ] 202012
• [ ] 202205
• [ ] 202305
• [ ] 202311
• [x] 202405

Approach

What is the motivation for this PR?

Investigating the cause of sonic-mgmt tacacs test fails on 202405, determined it was caused by duplicate user entry in tacplus config.

How did you do it?

How did you verify/test it?

After this change, all the tacacs sonic-mgmt tests pass for us in 202405.

Any platform specific information?

Supported testbed topology if it's a new test case?

Documentation