suricatactl: Make code compatible with Python 3

suricatactl: Make code compatible with Python 3

Call to suricatactl was failing with Python3 with the following error:
```
Traceback (most recent call last):
  File "bin/suricatactl", line 40, in <module>
    sys.exit(main())
  File "./suricata/ctl/main.py", line 50, in main
    args.func(args)
AttributeError: 'Namespace' object has no attribute 'func'
```
Fix this by making it run with Py3 just like it does with Py2.

Closes redmine ticket #2793