default template: use ds_name instead of VAL[name]

VAL[name] is escaped, replacing / with _
For disk checks, this is confusing since /backup becomes _backup
ds_name should always be available so let's use that instead.

This fixes #4239

Change-Id: I391e73a634a03a4880f704ae0732e771e8652424
Fixed-by: Martin Kamijo <mk@op5.com> & Mikael Falkvidd <mfalkvidd@op5.com>

Signed-off-by: Mikael Falkvidd <mfalkvidd@op5.com>
![4239_after](https://cloud.githubusercontent.com/assets/893502/4271666/ee46ee34-3cda-11e4-976e-cd27561f683b.png)
![4239_before](https://cloud.githubusercontent.com/assets/893502/4271667/ee49fc96-3cda-11e4-8473-62ad4464f4ae.png)