[core] should be possible to configure user id into which supervisor drop privs

First order of business would be to get rid of hardcoded hue in desktop/core/src/desktop/supervisor.py

# User and group to setuid/setgid down to for any supervisees that don't have
# the drop_root option set to False
SETUID_USER = "hue"
SETGID_GROUP = "hue"

If we could also make it configurable on the per-supervisee basis that would be terrific!