Limit the size/number of labels for TaskInfo/ExecutorInfo.

Currently framework can set arbitrary labels in TaskInfo/ExecutorInfo. However, some faulty framework (due to potential bugs) may set a huge number of labels or make the label size huge. It could make mess up the master http endpoints (e.g., /master/state.json or /master/state-summary), which causes the master being stuck or super slow.

We should make a configurable limit to the number/size of labels so that a problematic framework would not ruin the mesos cluster.