feat: use new group and var to install dashd_nightly_image on subset of nodes

[strophy]

Issue being fixed or feature implemented

Core team required latest-nightly images on some nodes

What was done?

Add condition to use dashd_nightly_image on nightly_masternodes

How Has This Been Tested?

On testnet

Breaking Changes

Weird new behaviour?

Checklist:

• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have added or updated relevant unit/integration/functional/e2e tests
• [x] I have made corresponding changes to the documentation

For repository code-owners and collaborators only

• [x] I have assigned this pull request to a milestone