Grafana-import

- Improved Python API authentication when using URL-based connectivity,
by respecting the `credential` keyword argument
- Added basic example program about how to import a dashboard
- Options: Permitted usage without authentication credentials or token
- Options: Do not assume defaults for `ACTION` and `--grafana_label` args
- Options: Print help text when using erroneous parameters

- Fixed folder argument issue
- Fixed import dashboards into a folder
- Added keep-uid argument to preserve the dashboard uid provided in file
- Added an option to import dashboards from a directory

Thanks, vrymar.

* Permit invocation without configuration file for ad hoc operations.
In this mode, the Grafana URL can optionally be defined using the
environment variable `GRAFANA_URL`.
* Fix exit codes in failure situations.
* Fix exception handling and propagation in failure situations.
* Add feature to support dashboard builders, in the spirit of
dashboard-as-code. Supports Grafonnet, grafana-dashboard, grafanalib,
and any other kind of executable program generating Grafana Dashboard
JSON.
* Add watchdog feature, monitoring the input dashboard for changes on
disk, and re-uploading it, when changed.
* Pass `GRAFANA_TOKEN` environment variable on Grafana initialization.
Thanks, jl2397.

* Migrated from grafana_api to grafana_client

* Fixed behavior for dashboard moved from one folder to another

* Added "remove dashboard" feature