Add admin get/set config keys API (#6113)

This PR adds two new admin APIs in Minio server and madmin package:
- GetConfigKeys(keys []string) ([]byte, error)
- SetConfigKeys(params map[string]string) (err error)

A key is a path in Minio configuration file, (e.g. notify.webhook.1)

The user will always send a string value when setting it in the config file,
the API will know how to convert the value to the appropriate type. The user
is also able to set a raw json.

Before setting a new config, Minio will validate all fields and try to connect
to notification targets if available.

vendor/vendor.json

@@ -888,6 +888,12 @@
 			"revision": "173748da739a410c5b0b813b956f89ff94730b4c",
 			"revisionTime": "2016-08-30T17:39:30Z"
 		},
+		{
+			"checksumSHA1": "j1wNJXkZyuFKjYFpPawESOaXYxk=",
+			"path": "github.com/tidwall/sjson",
+			"revision": "6a22caf2fd45d5e2119bfc3717e984f15a7eb7ee",
+			"revisionTime": "2016-12-12T16:53:56Z"
+		},
 		{
 			"checksumSHA1": "MWqyOvDMkW+XYe2RJ5mplvut+aE=",
 			"path": "github.com/ugorji/go/codec",