Adding YAML Import-Export for Datasources to CLI (#3978)

* Adding import and export for databases

* Linting

docs/import_export_datasources.rst

@@ -0,0 +1,103 @@
+Importing and Exporting Datasources
+===================================
+
+The superset cli allows you to import and export datasources from and to YAML.
+Datasources include both databases and druid clusters. The data is expected to be organized in the following hierarchy: ::
+
+    .
+    ├──databases
+    |  ├──database_1
+    |  |  ├──table_1
+    |  |  |  ├──columns
+    |  |  |  |  ├──column_1
+    |  |  |  |  ├──column_2
+    |  |  |  |  └──... (more columns)
+    |  |  |  └──metrics
+    |  |  |     ├──metric_1
+    |  |  |     ├──metric_2
+    |  |  |     └──... (more metrics)
+    |  |  └── ... (more tables)
+    |  └── ... (more databases)
+    └──druid_clusters
+       ├──cluster_1
+       |  ├──datasource_1
+       |  |  ├──columns
+       |  |  |  ├──column_1
+       |  |  |  ├──column_2
+       |  |  |  └──... (more columns)
+       |  |  └──metrics
+       |  |     ├──metric_1
+       |  |     ├──metric_2
+       |  |     └──... (more metrics)
+       |  └── ... (more datasources)
+       └── ... (more clusters)
+
+
+Exporting Datasources to YAML
+-----------------------------
+You can print your current datasources to stdout by running: ::
+
+    superset export_datasources
+
+
+To save your datasources to a file run: ::
+
+    superset export_datasources -f <filename>
+
+
+By default, default (null) values will be omitted. Use the ``-d`` flag to include them.
+If you want back references to be included (e.g. a column to include the table id
+it belongs to) use the ``-b`` flag.
+
+Alternatively you can export datasources using the UI: ::
+
+1. Open **Sources** -> **Databases** to export all tables associated to a single or multiple databases. (**Tables** for one or more tables, **Druid Clusters** for clusters, **Druid Datasources** for datasources)
+2. Select the items you would like to export
+3. Click **Actions** -> **Export to YAML**
+4. If you want to import an item that you exported through the UI, you will need to nest it inside its parent element, e.g. a `database` needs to be nested under `databases` a `table` needs to be nested inside a `database` element.
+
+Exporting the complete supported YAML schema
+--------------------------------------------
+In order to obtain an exhaustive list of all fields you can import using the YAML import run: ::
+
+    superset export_datasource_schema
+
+Again, you can use the ``-b`` flag to include back references.
+
+
+Importing Datasources from YAML
+-------------------------------
+In order to import datasources from a YAML file(s), run: ::
+
+    superset import_datasources -p <path or filename>
+
+If you supply a path all files ending with ``*.yaml`` or ``*.yml`` will be parsed.
+You can apply additional falgs e.g.: ::
+
+    superset import_datasources -p <path> -r
+
+Will search the supplied path recursively.
+
+The sync flag ``-s`` takes parameters in order to sync the supplied elements with
+your file. Be careful this can delete the contents of your meta database. Example:
+
+   superset import_datasources -p <path / filename> -s columns,metrics
+
+This will sync all ``metrics`` and ``columns`` for all datasources found in the
+``<path / filename>`` in the Superset meta database. This means columns and metrics
+not specified in YAML will be deleted. If you would add ``tables`` to ``columns,metrics``
+those would be synchronised as well.
+
+
+If you don't supply the sync flag (``-s``) importing will only add and update (override) fields.
+E.g. you can add a ``verbose_name`` to the the column ``ds`` in the table ``random_time_series`` from the example datasets
+by saving the following YAML to file and then running the ``import_datasources`` command. ::
+
+    databases:
+    - database_name: main
+      tables:
+      - table_name: random_time_series
+        columns:
+        - column_name: ds
+          verbose_name: datetime
+