Add process of reconnect to Nagios Server(NDOUtils/MySQL).

Currently, when disconnect occurs, not reconnect to Nagios Server.
Reconnect after recovery, In order to solve the disconnect event.
Currently DBAgentMySQL::Impl::connected is not set to false, even when the connection is lost.
I think a simple and safe way is to delete ArmNagiosNDOUtils::Impl::dbAgent and set it to NULL when the some trouble including exceptions happens.