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ros::NodeHandle node_handle("/move_group/arm_control");
...
node_handle.getParam("planning_plugin", ...);

and:

Launching executable with rosrun arm_control arm_control_node prints [FATAL] [1508419005.957461571]: Could not find planner plugin name. However rosparam get /move_group/planning_plugin outputs ompl_interface/OMPLPlanner.

Parameter lookups are relative to the ros::NodeHandle that you do them with.

In this case you ask for planning_plugin with a NodeHandle that you created in the /move_group/arm_control namespace. So in the end, node_handle will try to retrieve /move_group/arm_control/planning_plugin for you.

That parameter doesn't exist, hence the error.

Do you have a specific reason for creating your main ros::NodeHandle in that namespace?

It is perfectly ok to create NodeHandles in namespaces like that, but especially the "first one" is typically created without any namespace declaration at all.