Register the HTML formatter as the default
Register the HTML formatter as the default

This extends Avocado's default formatter to support an HTML output which
is ensures the /data/preview/ endpoint always returns a string output.

Signed-off-by: Byron Ruth <e9d71f5ee7c92d6dc9e92ffdad17b8bd49418f98@devel.io>

chop-dbhi/serrano,rv816/serrano_night,chop-dbhi/serrano,rv816/serrano_night