api : Fix template-show partial URL redirect

All other API actions will provide a redirect if you only provide the
stack ID or name (rather than both), the template URL currently doesn't
because the template subpath is missing from the mapper lookup

Fixes bug #1178705

Change-Id: Ib8e8dd7691bc48587c9d91e937ac2c2c35ac3ce6
Merge "api : Fix template-show partial URL redirect"