[digitalocean] Check to see if digitalocean_api_key exists before running exit code

When running a tests without mocking I get the following exception:

/Users/kyle.rames/Projects/fog/lib/fog/core/service.rb:208:in `validate_options': Missing required arguments: digitalocean_api_key, digitalocean_client_id (ArgumentError)
	from /Users/kyle.rames/Projects/fog/lib/fog/core/service.rb:58:in `new'
	from /Users/kyle.rames/Projects/fog/lib/fog/compute.rb:44:in `new'
	from /Users/kyle.rames/Projects/fog/lib/fog/compute.rb:5:in `[]'
	from tests/digitalocean/helper.rb:4:in `service'
	from tests/digitalocean/helper.rb:36:in `block in <top (required)>'

This pull request checks to see if we are mocking and if a digitalocean_api_key before attempting to delete servers.