Don't require callers to define their own restart command

Fixes https://github.com/joeyAghion/opsworks_custom_env/issues/7.

In an [earlier commit](https://github.com/joeyAghion/opsworks_custom_env/commit/b213203daf193fd83b6644afd46cf388e610ff4e), I removed the direct dependency on Opsworks' `rails::configure` recipe in order to make this cookbook more portable to other layers. That made it difficult to use this cookbook.

With this change, the `opsworks_custom_env::configure` recipe can again be associated with the _Deploy_ event and callers shouldn't have to worry about defining a restart command separately.

If custom layers want to leverage the config-writing without the unicorn restart, they can reference the `opsworks_custom_env::write_config` recipe directly and define their own restart command.