Set heat::instance_user to empty string

Set heat::instance_user to empty string

In the overcloud heat, heat.conf instance_user is set to heat-admin.

The consequence of this is that SSHing into heat created guest VMs will require
the user 'heat-admin'. I predict that this will result in user confusion as to
how to SSH into their VMs since they will be attempting default usernames
(centos, cloud-user etc) or the documented heat default user (ec2-user)

This change sets it to an empty string so that default usernames are used.

This change depends on the puppet-heat fix to allow empty string instance_user:
Depends-On: I9e8be0dd50709d271fc81683770c78380724e405

Change-Id: Id14bf3a4ac1b1c95797dae16c674b32a2da230f8