Have you restarted Galaxy since making the change to the galaxy.yml file?
Also, for this line, the option is not used unless the hash # is removed. Make sure not to add a space to the start of the line when doing this This changed or I understood it wrong originally – the line should start with an indent (replace the # with an indent) to preserve yaml formatting. See @marten’s reply below.
Related Q&A setting up as administrator - #5 by MzwaneleN (now closed out)