[JENKINS-19880] Pass region specified in web interface to the key generation Java method.

The [code](https://github.com/jenkinsci/ec2-plugin/blob/ec2-1.21/src/main/java/hudson/plugins/ec2/AmazonEC2Cloud.java#L152-L155) that is responsible for key generation requires region to be set. Unfortunately web UI has a bug that does not set that key.

This PR finally fixes the issue and get working.