Setting prompt for MariaDB 10.2 in my.cnf?
across all our database servers for everyone


This is a function of the client. You can't set it on the server. MySQL ships with a client, and a server. The client determines the prompt. There is nothing you can do on the server to fix that. The client could be something totally different that has for its prompt 

L33tHAX\###-&gt;#


You can see the code here construct_prompt(). Nothing there hits the server. That said, it gets set here in the main

default_prompt = my_strdup(getenv("MYSQL_PS1") ? 
                 getenv("MYSQL_PS1") : 
                 "\\N [\\d]&gt; ",MYF(MY_WME));
current_prompt = my_strdup(default_prompt,MYF(MY_WME));


So it would seem you could set it through the MYSQL_PS1 environmental variable.