MaxScale from v2.5 has a nice web Gui based on REST API. However it has default credentials admin:mariadb. Does any one know how to change this? I've searched around in Mariadb Docs, but found the only options you can change is to add ssl cert's for the web gui to encrypt passing passwords and switch off\on the auth at all. Any ideas?
You can use the maxctrl command line client to manage users. The MaxScale administration tutorial has a section on how to create new users and to remove old ones.
The REST API tutorial also has a section on how to harden it for a more secure installation.