Implement logs model using Elasticsearch

- Implement logs model using Elasticsearch with tests
- Implement transition model using both elasticsearch and database model
- Add LOGS_MODEL configuration to choose which to use.

Co-authored-by: Sida Chen <sidchen@redhat.com>
Co-authored-by: Kenny Lee Sin Cheong <kenny.lee@redhat.com>

test/test_api_usage.py

@@ -3391,6 +3391,7 @@ class TestOrgRobots(ApiTestCase):
     self.assertEquals(json['token'], json2['token'])
 
 
+# TODO(sidchen): Test logging to elastic search log model.
 class TestLogs(ApiTestCase):
   def test_repo_logs(self):
     self.login(ADMIN_ACCESS_USER)
@@ -3971,6 +3972,7 @@ class TestUserAuthorizations(ApiTestCase):
                          expected_code=404)
 
 
+# TODO(sidchen): Test logging to elastic search log model.
 class TestSuperUserLogs(ApiTestCase):
   def test_get_logs(self):
     self.login(ADMIN_ACCESS_USER)