Align `username` vs `dpu_bmc_username` everywhere

[glimchb]

also dpu_bmc_password and dpu_bmc_token

example declares username argument
https://github.com/opiproject/ansible-opi-dpu/blob/461d6a8a947a01d17d60168b0f8a7adbf3940229/roles/bmc_reboot/meta/argument_specs.yml
but uses dpu_bmc_username instead
https://github.com/opiproject/ansible-opi-dpu/blob/461d6a8a947a01d17d60168b0f8a7adbf3940229/roles/bmc_reboot/tasks/main.yml

[MAVRICK-1]

@glimchb can you assign me this issue

[glimchb]

@MAVRICK-1 let's see firsat how you do on your first PR #90