Make it possible to use boto3_conn outside modules

The `boto3_conn` function requires a module argument, and calls
`module.fail_json` if the connection doesn't receive enough arguments.
In non-module settings like inventory scripts, there is no module to be
passed.

The `boto3_inventory_conn` function takes the same arguments except for
`module`, and both call _boto3_conn which doesn't require a module be
passed.

lib/ansible/module_utils/ec2.py

@@ -55,10 +55,19 @@ class AnsibleAWSError(Exception):
 
 
 def boto3_conn(module, conn_type=None, resource=None, region=None, endpoint=None, **params):
+    try:
+        return _boto3_conn(conn_type=None, resource=None, region=None, endpoint=None, **params)
+    except ValueError:
+        module.fail_json(msg='There is an issue in the code of the module. You must specify either both, resource or client to the conn_type parameter in the boto3_conn function call')
+
+def _boto3_conn(conn_type=None, resource=None, region=None, endpoint=None, **params):
     profile = params.pop('profile_name', None)
 
     if conn_type not in ['both', 'resource', 'client']:
-        module.fail_json(msg='There is an issue in the code of the module. You must specify either both, resource or client to the conn_type parameter in the boto3_conn function call')
+        raise ValueError('There is an issue in the calling code. You '
+                         'must specify either both, resource, or client to '
+                         'the conn_type parameter in the boto3_conn function '
+                         'call')
 
     if conn_type == 'resource':
         resource = boto3.session.Session(profile_name=profile).resource(resource, region_name=region, endpoint_url=endpoint, **params)
@@ -71,6 +80,7 @@ def boto3_conn(module, conn_type=None, resource=None, region=None, endpoint=None
         client = boto3.session.Session(profile_name=profile).client(resource, region_name=region, endpoint_url=endpoint, **params)
         return client, resource
 
+boto3_inventory_conn = _boto3_conn
 
 def aws_common_argument_spec():
     return dict(