Feature/refactor service mixin pool declaration

Modifies the way ServiceMixinPool should be declared to : 

```python
from mangrove.pool import ServiceMixinPool

class MyMixinPool(ServiceMixinPool):
    _aws_services = {
        's3': {
            'regions': '*',
            'default_region': 'eu-west-1',
        },
        'ec2': {
            'regions': ['us-east-1', 'eu-west-1'],
            'default_region': 'eu-west-1'
        }
    }
``` 


Please, review @ggreg