I'd like to have Deployment Project variables that work like Build Plan Project variables (BAM-7132) added in v7.1.0.

In other words, I'd like to define variables at the Deployment Project scope that apply to all Environments.

Desired behavior:

1. Define variables at the Deployment Project scope that apply to all Environments
2. Support this feature in Bamboo Specs
3. For Specs-based Deployment Projects, allow UI access to add additional variables – this is important to allow sensitive values to be kept out of version control, even in an encrypted form (See BAM-18932). This is also consistent with behavior for Build Plan Projects and Specs-based Build Plans.