Remove unused packer/terraform tasks from environments

[disaac]

Currently if an environment is introduced that leverages just packer or terraform in its stacks, both task types will be displayed for that environment even though there isn't a stack implementing the task.

Expected:
Packer and terraform tasks should only be displayed at the environment level when an associated stack is implementing the task. In the absence of a supporting stack in the environment the task should be removed.

Reference: environment_tasks.rb