MM-25238: Fix system ping check to read from master (#14580)

The code was writing to master and immediately after that reading
from a replica causing it to fail intermittently.

Since this is not a very high-traffic table, it should be safe to read
from master always.

store/sqlstore/system_store.go

@@ -37,7 +37,7 @@ func (s SqlSystemStore) Save(system *model.System) *model.AppError {
 }
 
 func (s SqlSystemStore) SaveOrUpdate(system *model.System) *model.AppError {
-	if err := s.GetReplica().SelectOne(&model.System{}, "SELECT * FROM Systems WHERE Name = :Name", map[string]interface{}{"Name": system.Name}); err == nil {
+	if err := s.GetMaster().SelectOne(&model.System{}, "SELECT * FROM Systems WHERE Name = :Name", map[string]interface{}{"Name": system.Name}); err == nil {
 		if _, err := s.GetMaster().Update(system); err != nil {
 			return model.NewAppError("SqlSystemStore.SaveOrUpdate", "store.sql_system.update.app_error", nil, err.Error(), http.StatusInternalServerError)
 		}
@@ -71,7 +71,7 @@ func (s SqlSystemStore) Get() (model.StringMap, *model.AppError) {
 
 func (s SqlSystemStore) GetByName(name string) (*model.System, *model.AppError) {
 	var system model.System
-	if err := s.GetReplica().SelectOne(&system, "SELECT * FROM Systems WHERE Name = :Name", map[string]interface{}{"Name": name}); err != nil {
+	if err := s.GetMaster().SelectOne(&system, "SELECT * FROM Systems WHERE Name = :Name", map[string]interface{}{"Name": name}); err != nil {
 		return nil, model.NewAppError("SqlSystemStore.GetByName", "store.sql_system.get_by_name.app_error", nil, err.Error(), http.StatusInternalServerError)
 	}