Support EBS encryption in BlockDeviceType.

Support an optional configuration of ebs encryption in the
BlockDeviceType (which is used to build a BlockDeviceMapping).

The encrypted flag is optional (defaulting to `None`), as the AWS
API doesn't support any value for this flag for the root device
of a new instance.
Merge pull request #2587 from jcrobak/block-device-mapping-ebs-encrypted

Support EBS encryption in BlockDeviceType. Fixes #2587, #2480.