Implement `has_env' condition keyword

lib/producer/core/condition/dsl.rb

@@ -16,6 +16,8 @@ module Producer
           end
         end
 
+        define_test :has_env, Tests::HasEnv
+
         attr_accessor :tests
 
         def initialize(env, &block)

lib/producer/core/tests/has_env.rb

@@ -0,0 +1,11 @@
+module Producer
+  module Core
+    class Tests
+      class HasEnv < Test
+        def success?
+          env.remote.environment.has_key? arguments.first.to_s.upcase
+        end
+      end
+    end
+  end
+end