Permit specification of host in stunnel templates

While we can in principle do this already, there is a bug (1344284)
in heat which breaks our ability to calculate an appropriate unified
line from within heat. Instead, we can pass separate metadata to
os-apply-config and avoid this bug (which has no straight forward
fix).

Change-Id: I820862982afee07d85f1f0ad02b08cd5b0f35e49
Related-Bug: #1344284
Merge "Permit specification of host in stunnel templates"