Set stack_user_domain config correctly

The recently merged patch which creates a domain for heat
fails to correctly set the domain ID in heat.conf, so move the
setting of the config option to immediately after we create the
domain.  Also add the missing DEFAULT section identifier in the
iniset, and use OS_TOKEN instead of OS_SERVICE token, because
the stack.sh comment says this is exported for the openstackclient
workaround.

Change-Id: I912f774f1215d68cbcfe44229b371f318d92966a
Closes-Bug: #1283075
Merge "Set stack_user_domain config correctly"