Clean up nginx_proxy recipe

Consistency is key

* Align resource attributes’ arguments
* Use strings for modes
Move url_version attribute into recipe

Dynamically assigned attributes like these are best
set inside recipes.

In this way, we could change the version attributes
in a wrapper cookbook, and not need to also set
this attribute.
Clean up default recipe

* Align resources’ attributes’ arguments
* Remove potentially inaccurate comment
    * Depending on attribute settings, it might not be
installed in `/opt`
Upgrade to Archiva 2.1.0