Allow for overriding user and data directory

This allows for overriding the user that runs elasticsearch (needed for the `ps` grep stuff so it works for users other than `elasticsearch`) and for overriding the directory where the index data is kept.
Syntax in the config file is `env.es_user <<username>>`.

Running elasticsearch 1.3.x and 1.4.x, the grep for `es.default.path.data` didn't yield anything so I added an option to allow for overriding the data directory in the config via eg. `env.data_dir /data/index/`