Ec2 eu central

This adds support to for the eu-central-1 region (ec2 only). This requires the following changes:

- adding eu-central-1 to the whitelist of regions (I noticed some services had their own whitelist - I assume this is an artifact of times when certain services only existed in some regions)
- switching over to user aws signature v4

All AWS regions support AWS signature v4 (new regions, such as eu-central-1 only support v4), so this won't break anything for existing AWS users. I'm less sure about the impact on people using services with an AWS compatible API (eg eucalyptus) - while I assume current versions support v4, presumably some people out there still use older versions.

If we can agree the approach (eg should we allow people to request signature v2?) then it should be straightforward to make this change to the other aws services