Allow per-deployment configuration of user id

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
Merge "Allow per-deployment configuration of user id"