Error occurred in awslog group

My goal is to refer to the log with 'awslogs get -w '.

```
$ aws --version
aws-cli/1.11.132 Python/2.7.12 Linux/4.9.58-18.55.amzn1.x86_64 botocore/1.8.16
$ awslogs groups

================================================================================
You've found a bug! Please, raise an issue attaching the following traceback
https://github.com/jorgebastida/awslogs/issues/new
--------------------------------------------------------------------------------
Version: 0.10.0
Python: 2.7.12 (default, Nov 2 2017, 19:20:38)
[GCC 4.8.5 20150623 (Red Hat 4.8.5-11)]
boto3 version: 1.5.2
Platform: Linux-4.9.58-18.55.amzn1.x86_64-x86_64-with-glibc2.2.5
Config: {'func': 'list_groups', 'aws_region': None, 'log_group_prefix': None, 'aws_access_key_id': 'SENSITIVE', 'aws_profile': 'SENSITIVE', 'aws_secret_access_key': 'SENSITIVE', 'aws_session_token': 'SENSITIVE'}
Args: ['/usr/local/bin/awslogs', 'groups']

Traceback (most recent call last):
File "/usr/local/lib/python2.7/site-packages/awslogs/bin.py", line 168, in main
logs = AWSLogs(**vars(options))
File "/usr/local/lib/python2.7/site-packages/awslogs/core.py", line 62, in __init__
region_name=self.aws_region
File "/usr/local/lib/python2.7/site-packages/boto3/__init__.py", line 83, in client
return _get_default_session().client(*args, **kwargs)
File "/usr/local/lib/python2.7/site-packages/boto3/session.py", line 263, in client
aws_session_token=aws_session_token, config=config)
File "/usr/local/lib/python2.7/site-packages/botocore/session.py", line 861, in create_client
client_config=config, api_version=api_version)
File "/usr/local/lib/python2.7/site-packages/botocore/client.py", line 76, in create_client
verify, credentials, scoped_config, client_config, endpoint_bridge)
File "/usr/local/lib/python2.7/site-packages/botocore/client.py", line 288, in _get_client_args
verify, credentials, scoped_config, client_config, endpoint_bridge)
File "/usr/local/lib/python2.7/site-packages/botocore/args.py", line 45, in get_client_args
endpoint_url, is_secure, scoped_config)
File "/usr/local/lib/python2.7/site-packages/botocore/args.py", line 111, in compute_client_args
service_name, region_name, endpoint_url, is_secure)
File "/usr/local/lib/python2.7/site-packages/botocore/client.py", line 361, in resolve
service_name, region_name)
File "/usr/local/lib/python2.7/site-packages/botocore/regions.py", line 122, in construct_endpoint
partition, service_name, region_name)
File "/usr/local/lib/python2.7/site-packages/botocore/regions.py", line 135, in _endpoint_for_partition
raise NoRegionError()
NoRegionError: You must specify a region.
================================================================================
```

Start at awslogs/core.py line 62 and reproduce the failure with `awslogs groups`, using the reported traceback and AWS CLI details. Trace how the command obtains its region and define completion as making this invocation handle the missing-region condition without the shown NoRegionError.

Written by the indexing model from the issue text.