Added cross-platform uptime fact

Having an uptime fact allows you to check after a reboot whether the system was effectively rebooted.

```yaml
  - name: Safeguard - Was system properly rebooted ?
    action: fail msg="System was not properly rebooted"
    when: ansible_uptime > 900
```

This patch has been tested on Linux, Solaris and HP-UX.
Update facts.py
Merge pull request #7670 from dagwieers/patch-3

Added cross-platform uptime fact