This commit eliminates the following warning (of which there are currently dozens):
../boto/ec2/connection.py:582:
PendingDeprecationWarning: The current get_all_instances implementation will be replaced with get_all_reservations.
`boto` isn't likely to ever make good on this warning, but doing the replacement will
declutter the `moto` test output.
Boto can be configured with extra regions, but moto will fail to import
if they are not in the hardcoded list in ec2/models.py. Instead, use the
region list from boto to build the ec2_backends dict to ensure all
regions are available.
