Correct redfish_exporter_default_username

`ipmi_user` is not something we define by default. Changes to use
`ipmi_username`, which also matches our docs.

Author: MoteHue

etc/kayobe/stackhpc-monitoring.yml

@@ -49,7 +49,7 @@ stackhpc_os_capacity_openstack_verify: true
 stackhpc_enable_redfish_exporter: false
 
 # Credentials
-redfish_exporter_default_username: "{{ ipmi_user }}"
+redfish_exporter_default_username: "{{ ipmi_username }}"
 redfish_exporter_default_password: "{{ ipmi_password }}"
 
 # The address of the BMC that is used to query redfish metrics.