Make the deployment role configuration as default

[spacewander]

Description

Currently, there are two ways to configure APISIX's data source / enable Admin API or not:

1. via various configurations, like enable_admin \ config_center \ etcd.
2. via deployment role: https://github.com/apache/apisix/blob/master/docs/en/latest/architecture-design/deployment-role.md

In APISIX v3, we are going to make the deployment role the default.

Maybe we can remove those various configurations like enable_admin \ config_center \ etcd at that time.