Pick up default EC2 region from boto.config

This doesn't account for boto configs where e.g. RDS has one
default region and EC2 another - all will default to `ec2_region_name`.

However, this is just handy to allow an easy site wide default
region if existing configuration already relies on it.

Modules can be improved to mention this in the documentation and
turn off required=True where needed. But it works with `ec2`
and `ec2_vol` without change.
Merge branch 'ec2_boto_config_region' of https://github.com/willthames/ansible into willthames-ec2_boto_config_region
Minor tweak to region config in boto configuration file