Use - instead of . for physical_resource_name delim.

Avoiding using . means that instance hostnames are unique within the stack
(and within the tenant).

This solves 2 real-world use cases:
- tripleo issue, as described in bug: #1184713
- OpenShift template resorts to setting the hostname to ec2_instance_id

This may have an impact on users who are scripting to assume . delimiting,
however this will only affect newly created stacks after this change is
deployed.

Fixes bug: #1184713

Change-Id: I5a8eb358ea9c52162eb9263b9f9e6dcf750d787b