Set the "disable default image" variable in os_profiles app settings.

#6420 

This fixes the disappearing profile images in some vsites. The problem was that not variable was even set in the submit of the app settings. Added a "set" for the variable still didn't solve this (that is, one vsite affects other vsites).
And so, the "dummy__" prefix although not optimal is used to make sure this setting hold only at the vsite level (this was also what we did in another os_profile app settings and was needed here as well).