Include env validation of new config

So we fail fast when required ENVs are missing!

test/configuration_test.rb

@@ -126,15 +126,17 @@ class ConfigurationTest < ActiveSupport::TestCase
   end
 
   test "env args with missing secret" do
-    config = Mrsk::Configuration.new(@deploy.tap { |c| c.merge!({
-      env: { "secret" => [ "PASSWORD" ] }
-    }) })
-
     assert_raises(KeyError) do
-      assert_equal [ "-e", "PASSWORD=secret123" ], config.env_args
+      config = Mrsk::Configuration.new(@deploy.tap { |c| c.merge!({
+        env: { "secret" => [ "PASSWORD" ] }
+      }) })
     end
   end
 
+  test "valid config" do
+    assert @config.valid?
+  end
+
   test "ssh options" do
     assert_equal "root", @config.ssh_options[:user]