Crowbar revisions and schema versions

proposal_schema_version is defined in each barclamp's crowbar.yml file. This is a global version which is applied to all barclamps. It is a sort of API versions for what barclamps support, it defines the feature level for that barclamp.

For example, the proposal_schema_version could be incremented when a new feature is added in barclamp-crowbar, which all barclamps would have access to, but they need to declare that they support that new API by bumping their proposal_schema_version first (and possibly making any required changes to their own schema).

schema-revision is defined in each barclamp's *.schema file and is specific to one barclamp. It is incremented when the schema file changes.