Added test to verify modified IPMI username
Added test to verify modified IPMI username

Test cases added:

    - Modify_IPMI_User

It will create a new user and set access, change previlege

and change the username and verify the modified username is accessible.

verifies modified IPMI user is communicating via IPMI.

Tested: Run robot "Modify_IPMI_User" of ipmi/test_ipmi_user.robot

Signed-off-by: Chithra Gopinathan  <2ca6f9000a278386a465e5dd9bece0fca05db71d@ami.com>
Change-Id: I712da488422084c857fb54fb70fdac7804add656

openbmc/openbmc-test-automation,openbmc/openbmc-test-automation