Added support for package version

[knowshan]

Thanks for creating this module!

I've added support for specifying package version if it helps. Sometimes you may have dependency on specific node_exporter version because of Grafana version. These changes will allow you to specify package version.

Example:

---
- hosts: example
  tasks:
    - include_role: name=prometheus_rpm 
      vars:
        - prometheus_rpm_components:
          - node_exporter: '0.14.0'
          - haproxy_exporter: ''