FrameworkInfo should include a Labels field to support arbitrary, lightweight metadata

A framework instance may offer specific capabilities to the cluster: storage, smartly-balanced request handling across deployed tasks, access to 3rd party services outside of the cluster, etc. These capabilities may or may not be utilized by all, or even most mesos clusters. However, it should be possible for processes running in the cluster to discover capabilities or features of frameworks in order to achieve a higher level of functionality and a more seamless integration experience across the cluster.

A rich discovery API attached to the FrameworkInfo could result in some form of early lock-in: there are probably many ways to realize cross-framework integration and external services integration that we haven't considered yet. Rather than over-specify a discovery info message type at the framework level I think FrameworkInfo should expose a *very generic* way to supply metadata for interested consumers (other processes, tasks, etc).

Adding a Labels field to FrameworkInfo reuses an existing message type and seems to fit well with the overall intent: attaching generic metadata to a framework instance. These labels should be visible when querying a mesos master's state.json endpoint.