Select node version

Fixes for issues: #305 and #254.
Moved node version selection from the node deployment logic in kudu service to the deployment script generator (when generating a [node deployment script](https://github.com/amitapl/azure-sdk-tools-xplat/commit/440cea45d7d2b6d03b75571cf25461b0e671d4ca)).
This way:
1. Custom deployment script has all the logic required for deploying (including node version selection).
2. npm is run using the selected node version.

Number 2 is only for the new generator deployment logic and not for the original one, since we will switch this sprint (or the next) and until then users can use the custom script if they require this support.