Add a require_destination setting

If you always want to use a destination, and have a base deploy.yml file
that doesn't specify any hosts, then if you forget to specific the
destination you will get a cryptic error.

Add a "require_destination" setting you can use to avoid this.

test/fixtures/deploy_for_required_dest.world.yml

@@ -0,0 +1,5 @@
+servers:
+  - 1.1.1.1
+  - 1.1.1.2
+env:
+  REDIS_URL: redis://x/y