refactor plugin_spec
WIP towards "change cf properties"
"change cf properties" updates deployment_attrs & deploys
verify that change properties are mutable
Clear terminology: attributes vs properties

* attributes are the simple variables users can change
* properties are the nested values passed into cf-release
set_mutable performs own mutability test
Add a FIXME
Refactor validation into DeploymentAttributes
"change cf attrs" also validates properties before deployment
Document how to scale up persistent disk
Merge pull request #188 from cloudfoundry-community/v0.7-change-props

change attributes with `$ bosh change cf attributes persistent_disk=8192`