Create database with separate script.

Creating the ATAT database requires a separate connection to one of the
default Postgres databases, like `postgres`. This updates the scripts
and secrets-tool command to handle creating the database. It also
removes database creation from Terraform and updates the documentation.

script/create_database.py

@@ -0,0 +1,41 @@
+# Add root application dir to the python path
+import os
+import sys
+
+parent_dir = os.path.abspath(os.path.join(os.path.dirname(__file__), ".."))
+sys.path.append(parent_dir)
+
+import sqlalchemy
+
+from atst.app import make_config
+
+
+def _root_connection(config, root_db):
+    # Assemble DATABASE_URI value
+    database_uri = "postgresql://{}:{}@{}:{}/{}".format(  # pragma: allowlist secret
+        config.get("PGUSER"),
+        config.get("PGPASSWORD"),
+        config.get("PGHOST"),
+        config.get("PGPORT"),
+        root_db,
+    )
+    engine = sqlalchemy.create_engine(database_uri)
+    return engine.connect()
+
+
+def create_database(conn, dbname):
+    conn.execute("commit")
+    conn.execute(f"CREATE DATABASE {dbname};")
+    conn.close()
+
+    return True
+
+
+if __name__ == "__main__":
+    dbname = sys.argv[1]
+    config = make_config()
+
+    conn = _root_connection(config, "postgres")
+
+    print(f"Creating database {dbname}")
+    create_database(conn, dbname)