Recognize `encrypted` keyword for ec2 volumes

- added for both `create_volume()` and `create_attach_volumes()`
- requirements
  - setting provider profile `ec2_api_version` to `2014-10-01`
  - using one of the supported instance types listed in http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/EBSEncryption.html#EBSEncryption_supported_instances
- announcment:
  - https://aws.amazon.com/about-aws/whats-new/2014/05/21/Amazon-EBS-encryption-now-available/