skip db creation in init if already exist (Netflix#142)

In some PaaS like Platform.sh, you already start with an existing
database and the user doesn't have permission to create new ones.
Only to create/delete tables.

Author: diogomatsubara

src/dispatch/cli.py

@@ -489,9 +489,10 @@ def database_trigger_sync():
 @dispatch_database.command("init")
 def init_database():
     """Initializes a new database."""
-    from sqlalchemy_utils import create_database
+    from sqlalchemy_utils import create_database, database_exists
 
-    create_database(str(config.SQLALCHEMY_DATABASE_URI))
+    if not database_exists(str(config.SQLALCHEMY_DATABASE_URI)):
+        create_database(str(config.SQLALCHEMY_DATABASE_URI))
     Base.metadata.create_all(engine)
     alembic_path = os.path.join(os.path.dirname(os.path.realpath(__file__)), "alembic.ini")
     alembic_cfg = AlembicConfig(alembic_path)