make minion numConcurrentTasksPerInstance configurable in config files

Currently, the default value of numConcurrentTasksPerInstance is 1 for all types of minion tasks. This default value is from `org.apache.helix.task.JobConfig.DEFAULT_NUM_CONCURRENT_TASKS_PER_INSTANCE`. Although one can use the /cluster/config API to configure it for each type of minion task, it is desirable that the default value can be configured in a config file, so that
1/ users do not need to configure it after the cluster is up.
2/ users can override it for all types of minion tasks.