Fix ec2 ModifyVolumeAttribute model

`AutoEnableIO` is AttributeBooleanValue type, not Boolean type.

Fixes aws/aws-cli#803
Sync with the latest ec2 model
Merge branch 'quiver-fix-ec2-modify-volume-attribute-model' into develop

* quiver-fix-ec2-modify-volume-attribute-model:
  Sync with the latest ec2 model
  Fix ec2 ModifyVolumeAttribute model