chore(rest): create subresources for process instance and variables

- allows reuse of common functionality for variables
Merge branch 'master' into rest_subresource_refactoring
chore(rest): create subresource for execution
refactor(rest): user String.format for variable error messages
chore(rest): create subresources for process definition and task
Merge branch 'rest_subresource_refactoring'