remove const qualifier form DataStoreManager::getDataStoreVector(),
because DataStore instance may be used not as a const.

For example, we will call DataStoreZabbix::getTriggers() and the
similar functions. They may change the internal state. that is,
they cannot be defined as const methods.

<< Tasks to do in the near future >>
(P) create DataStoreZabbix instance in mainRoutine().
(Q) remove code to craete ArmZabbixAPI instance in ArmController.
    The ArmZabbixAPI instance will be created in DataStoreZabbix,
    When the above (P) is implemented.
(R) pass command line argument for ArmZabbiXAPI such as IP address etc.
    to DataSotreZabbix.

<< Next milestone >>
showing a 'Overview' of ZABBIX PHP UI. The assumed design of
the data stream is the following.

[Zabbix Server] =>
  (A) ArmZabbAPI -> (B) DataStoreZabbix -> (C) VirtualDataStoreZabbix
    -> (D) SQLProcessorZabbix -> (D) FaceMySQLWorker  => ZABBIX PHP client

<< Next next milestone >>
Merge data from two zabbix servers at (C) VirtualDataStoreZabbix