Fix AWS EC2 external inventory script config read

EC2 inventory scripts reads configuration from an INI file. The `instance_filters` option controls which EC2 instances are retrieved for inventory. Filling this option and running the inventory script with Python 3 crashes with the following error:

```python
Traceback (most recent call last):
  File "./contrib/inventory/ec2.py", line 1328, in <module>
    Ec2Inventory()
  File "./contrib/inventory/ec2.py", line 163, in __init__
    self.read_settings()
  File "./contrib/inventory/ec2.py", line 393, in read_settings
    for instance_filter in config.get('ec2', 'instance_filters', '').split(','):
TypeError: get() takes 3 positional arguments but 4 were given
```

The problem is the last parameter of config.get() call, because `fallback` keyword argument is not specified.

The fix handles epmpty `instance_filers` in case of Python 2&3

contrib/inventory/ec2.py

@@ -388,7 +388,10 @@ class Ec2Inventory(object):
         # Instance filters (see boto and EC2 API docs). Ignore invalid filters.
         self.ec2_instance_filters = defaultdict(list)
         if config.has_option('ec2', 'instance_filters'):
-            for instance_filter in config.get('ec2', 'instance_filters', '').split(','):
+
+            filters = [tag for tag in config.get('ec2', 'instance_filters').split(',') if tag]
+
+            for instance_filter in filters:
                 instance_filter = instance_filter.strip()
                 if not instance_filter or '=' not in instance_filter:
                     continue