Fix config leaks and deprecate file-based config setters in NAS gateway (#9884)

This PR has the following changes

- Removing duplicate lookupConfigs() calls.
- Deprecate admin config APIs for NAS gateways. This will avoid repeated reloads of the config from the disk.
- WatchConfigNASDisk will be removed
- Migration guide for NAS gateways users to migrate to ENV settings.

NOTE: THIS PR HAS A BREAKING CHANGE

Fixes #9875

Co-authored-by: Harshavardhana <harsha@minio.io>

docs/gateway/nas.md

@@ -49,6 +49,36 @@ mc ls mynas
 [2017-02-26 22:10:11 PST]     0B test-bucket1/
 ```
 
+## Breaking changes
+
+There will be a breaking change after the release version 'RELEASE.2020-06-22T03-12-50Z'.
+
+### The file-based config settings are deprecated in NAS
+
+The support for admin config APIs will be removed. This will include getters and setters like `mc admin config get` and `mc admin config`  and any other `mc admin config` options. The reason for this change is to avoid un-necessary reloads of the config from the disk. And to comply with the Environment variable based settings like other gateways.
+
+### Migration guide
+
+The users who have been using the older config approach should migrate to ENV settings by setting environment variables accordingly.
+
+For example,
+
+Consider the following webhook target config.
+
+```
+notify_webhook:1 endpoint=http://localhost:8080/ auth_token= queue_limit=0 queue_dir=/tmp/webhk client_cert= client_key=
+```
+
+The corresponding environment variable setting can be
+
+```
+export MINIO_NOTIFY_WEBHOOK_ENABLE_1=on
+export MINIO_NOTIFY_WEBHOOK_ENDPOINT_1=http://localhost:8080/
+export MINIO_NOTIFY_WEBHOOK_QUEUE_DIR_1=/tmp/webhk
+```
+
+> NOTE: Please check the docs for the corresponding ENV setting. Alternatively, We can obtain other ENVs in the form `mc admin config set alias/ <sub-sys> --env`
+
 ## Explore Further
 - [`mc` command-line interface](https://docs.min.io/docs/minio-client-quickstart-guide)
 - [`aws` command-line interface](https://docs.min.io/docs/aws-cli-with-minio)