Replace #to_h with #to_hash for block device mappings
Replace #to_h with #to_hash for block device mappings

The object is either a Hash _like_ object that responds to #to_hash, or
a Hash. #to_h is only defined on the former. This change will eliminate
a NoMethodError that surprises people.

Signed-off-by: Jesse Zhang <4e375e814be137a4f94f9d2b2a5f4ca930cc7a5a@pivotallabs.com>

cloudfoundry-incubator/bosh-aws-cpi-release,tracyde/bosh-aws-cpi-release,wcamarao/bosh-aws-cpi-release,wcamarao/bosh-aws-cpi-release,wcamarao/bosh-aws-cpi-release,jfuerth/bosh-aws-cpi-release,jfuerth/bosh-aws-cpi-release,jfuerth/bosh-aws-cpi-release,tracyde/bosh-aws-cpi-release,cloudfoundry-incubator/bosh-aws-cpi-release,cloudfoundry-incubator/bosh-aws-cpi-release,tracyde/bosh-aws-cpi-release