Previously user ids of new instances were limited to ec2-user.
This patch adds a new configuration option to be placed in
/etc/heat/heat-engine.conf called "default_instance_user" which
allows the default of ec2-user to be overriden.
Note for reviewers that runcmd does not work properly. It was
actually running after the loguserdata.py script finished execution.
Fixes: Bug #1101347
Change-Id: Ica2dbe63d9dcbce8bb8de298eba452c34ab173d9
ac8b8f9c42