Parameterize package names

This adds parameters for the following packages:
* node_pkg
* npm_pkg
* dev_pkg

For backwards-compatibility, I have left the `dev_package` boolean name
intact.

Added a little more documentation.

Noted that this has been tested on Red Hat Enterprise Linux 6.

Update tests:
* Default to 'false' for `dev_package` on RHEL and derivatives (including
Fedora and Amazon).
* Add tests for RHEL and derivatives:
  * when `node_pkg` is defined
  * when `node_pkg` and `npm_pkg` are defined