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

python/suricata/ctl/filestore.py

@@ -30,7 +30,6 @@ class InvalidAgeFormatError(Exception):
     pass
 
 def register_args(parser):
-
     parsers = parser.add_subparsers()
 
     prune_parser = parsers.add_parser("prune")

python/suricata/ctl/main.py

@@ -46,5 +46,8 @@ def main():
     filestore.register_args(subparsers.add_parser("filestore"))
 
     args = parser.parse_args()
-
-    args.func(args)
+    try:
+        func = args.func
+    except AttributeError:
+        parser.error("too few arguments")
+    func(args)

python/suricata/ctl/test_filestore.py

@@ -2,7 +2,7 @@ from __future__ import print_function
 
 import unittest
 
-import filestore
+from suricata.ctl import filestore
 
 class PruneTestCase(unittest.TestCase):