fix ambiguity of nginx['version'] for nginx::source

`node['nginx']['version']` is an automatic_attribute (via ohai plugin), and it is nil when this cookbook is first used.

At first run of the cookbook, empty `node['nginx']['version']` causes installed folder `/opt/nginx-` to be used by nginx::source instead of `/opt/nginx-1.0.14` (1.0.14 is the default version).

Later, when `node['nginx']['version']` is used on subsequent runs, its value is stored in ohai via the plugin. Changing the value in a role, or node override attribute has no effect and will not upgrade the existing version.

This change creates a specific attribute called `node['nginx']['source']['version']`, used to force `nginx::source` to upgrade the version. And it prevents the conflict with the ohai automatic `node['nginx']['version']`.