Don't create cloud-init user unless specified

When the the instance_user value from heat.conf is set to empty string/None and
the user doesn't specify Server's admin_user property, Heat will not create a
custom cloud-init user.

The instance_user config option and admin_user property are deprecated and will
be removed in Juno where this behaviour becomes the default.

AWS::EC2::Instance will still create a cloud-init user for CloudFormation
compatibility. In the absence of the instance_user config option, 'ec2-user'
will be used.

Closes-Bug: #1257410
Change-Id: I42dda36045f79be079b2030669284e9db71463d7
Merge "Don't create cloud-init user unless specified"