Honor envs properly for access and secret key. (#3703)

Also changes the behavior of `secretKeyHash` which is
not necessary to be sent over the network, each node
has its own secretKeyHash to validate.

Fixes #3696
Partial(fix) #3700 (More changes needed with some code cleanup)

cmd/admin-handlers_test.go

@@ -43,6 +43,7 @@ type adminXLTestBed struct {
 func prepareAdminXLTestBed() (*adminXLTestBed, error) {
 	// reset global variables to start afresh.
 	resetTestGlobals()
+
 	// Initialize minio server config.
 	rootPath, err := newTestConfig(globalMinioDefaultRegion)
 	if err != nil {