When launching workers as separate tasks, it can be useful to be able to
use additional logic to define environment variables. This commit
provides an abstraction to allow for the customization of environment
variables on a per worker basis as well as two useful implementations:
* A no-op implementation (returns an empty Map).
* An implementation that moves the current environment variable handling
* out of the `DeployerPartitionHandler`.
Resolvesspring-cloud/spring-cloud-task#181
526dc41af9