Add IAM groups support (#7981)

This change adds admin APIs and IAM subsystem APIs to:

- add or remove members to a group (group addition and deletion is
  implicit on add and remove)

- enable/disable a group

- list and fetch group info

cmd/sts-handlers.go

@@ -183,13 +183,18 @@ func (sts *stsAPIHandlers) AssumeRole(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	policyName, err := globalIAMSys.PolicyDBGet(user.AccessKey)
+	policies, err := globalIAMSys.PolicyDBGet(user.AccessKey, false)
 	if err != nil {
 		logger.LogIf(ctx, err)
 		writeSTSErrorResponse(w, stsErrCodes.ToSTSErr(ErrSTSInvalidParameterValue))
 		return
 	}
 
+	policyName := ""
+	if len(policies) > 0 {
+		policyName = policies[0]
+	}
+
 	// This policy is the policy associated with the user
 	// requesting for temporary credentials. The temporary
 	// credentials will inherit the same policy requirements.