metadata: Switching to use .catkin_tools directory

- Metadata is stored in a `.catkin_tools` directory with a simple README
  file
- Individual yaml files are used for verb-sepcific metadata. I.e.
  `catkin build` creates/updates the file `.catkin_tools/build.yml`
- When initializing the metadata directory, the metadata module checks
  to see if that directory is contained within another
  catkin_tools-based workspace.