Improve examples & related tests (#7773)

* [WiP] improve load_examples

related to #7472, longer term we will generate the examples by exporting
them into tarball as in #7472. In the meantime, we need this subset of
the features:

* allowing specifying an alternate database connection for examples
* allowing a --only-metadata flag to `load_examples` to load only
  dashboard and chart definitions, no actual data is loaded

* Improve logging

* Rename data->examples

* Load only if not exist

* By default do not load, add a force flag

* fix build

* set published to true

tests/db_engine_specs_test.py

@@ -39,6 +39,7 @@ from superset.db_engine_specs.pinot import PinotEngineSpec
 from superset.db_engine_specs.postgres import PostgresEngineSpec
 from superset.db_engine_specs.presto import PrestoEngineSpec
 from superset.models.core import Database
+from superset.utils.core import get_example_database
 from .base_tests import SupersetTestCase
 
 
@@ -925,14 +926,14 @@ class DbEngineSpecsTestCase(SupersetTestCase):
         )  # noqa
 
     def test_column_datatype_to_string(self):
-        main_db = self.get_main_database()
-        sqla_table = main_db.get_table("energy_usage")
-        dialect = main_db.get_dialect()
+        example_db = get_example_database()
+        sqla_table = example_db.get_table("energy_usage")
+        dialect = example_db.get_dialect()
         col_names = [
-            main_db.db_engine_spec.column_datatype_to_string(c.type, dialect)
+            example_db.db_engine_spec.column_datatype_to_string(c.type, dialect)
             for c in sqla_table.columns
         ]
-        if main_db.backend == "postgresql":
+        if example_db.backend == "postgresql":
             expected = ["VARCHAR(255)", "VARCHAR(255)", "DOUBLE PRECISION"]
         else:
             expected = ["VARCHAR(255)", "VARCHAR(255)", "FLOAT"]