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.
Refactor the currently well-factored ec2 modules (i.e. those that already use ec2_connect) to
have a common argument spec. The idea is that new modules can use this spec without duplication
of code, and that new functionality can be added to the ec2 connection code (e.g. security
token argument)
