How to find out what is the developername value for a wave dashboard?
The easiest is on a dashboard to switch to JSON Editor via CRTL+E, at the top of the JSON you can see the developer name.
Looking at Dashboards List Resource you could use the REST API to iterate the available Wave Dashboards.
From the example response the keyprefix for a Wave Dashboard is 0FK. You could check the Partner API for the corresponding sObject. If it is exposed a SOQL query could also iterate the available dashboards.