Adding Bower, Gulp, and Grunt to the PATH

If there is a `<TOOLNAME>_PATH` specified, then use that, otherwise use the pre-installed one.
If there is a `<TOOLNAME>_VERSION` defined, use that, otherwise use the latest one.

No reason, I saw [`SystemEnvironment` used for node version](https://github.com/projectkudu/kudu/blob/master/Kudu.Core/Infrastructure/PathUtility.cs#L139), so I thought I would use the same style.