bug: should not save the runtime data of plugin into etcd

[nic-chen]

Issue description

admin api will pollute and save plugin_conf into etcd when calling plugin's function check_schema.
e.g. https://github.com/apache/incubator-apisix/blob/master/apisix/plugins/proxy-rewrite.lua#L111

Environment

• apisix version (cmd: apisix version): 1.4
• OS: centos7

Minimal test code / Steps to reproduce the issue

1. add a route with plugin proxy_write
2. view the route just added

What's the actual result? (including assertion message & call stack if applicable)

the runtime data of the plugin has be saved into etcd.

What's the expected result?

the runtime data of the plugin should't be saved into etcd.

[membphis]

only Admin API has this bug.

[wrl96]

I think we should remove apisix.admin.plugins.check_schema, just check schema in sync_data

[membphis]

@wrl96 Your way is dangerous, we don't allow dirty data to be stored in etcd.

[membphis]

I will fix this bug