Fix up hosted engine PXE in the model

Don't try to set a checkbox to "yes". Instead, update the
retrieve method to properly return a boolean, like every other
config.defaults class

Change-Id: I5b7e1fd75bacb077de7c44584953ef7fe5ea2304
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1151347
Signed-off-by: Ryan Barry <rbarry@redhat.com>