pybind/mgr/dashboard: build frontend/dist in build

pybind/mgr/dashboard: build frontend/dist in build

to enable us to build out of source, we should build the frontend
artifacts out of src. in this change:

* cmake:
  - install frontend/dist from build
  - drop rules to exclude unused frontend artifacts.
  - pass `--output-path` option to ng
  - copy `frontend/package.json` to build so that the frontend
    can find it
* pybind/mgr/dashboard/module.py: fall back to build directory
  if frontend/dist is not found.
* pybind/mgr/dashboard/__init__.py: use frontend/dist when performing
  unit test.

Signed-off-by: Kefu Chai <kchai@redhat.com>