non-string values in supervisor.scheduler.meta cause crash

The type of values in supervisor.scheduler.meta is not validated, but if one is not a string, supervisors crash during Thrift serialization with:

java.lang.ClassCastException: java.lang.Boolean cannot be cast to java.lang.String
at org.apache.storm.generated.SupervisorInfo$SupervisorInfoStandardScheme.write(SupervisorInfo.java:1241)
at org.apache.storm.generated.SupervisorInfo$SupervisorInfoStandardScheme.write(SupervisorInfo.java:1049)
at org.apache.storm.generated.SupervisorInfo.write(SupervisorInfo.java:923)
at org.apache.storm.thrift.TSerializer.serialize(TSerializer.java:79)
at org.apache.storm.serialization.GzipThriftSerializationDelegate.serialize(GzipThriftSerializationDelegate.java:40)
at org.apache.storm.utils.Utils.serialize(Utils.java:210)
at org.apache.storm.cluster.StormClusterStateImpl.supervisorHeartbeat(StormClusterStateImpl.java:419)
at org.apache.storm.daemon.supervisor.timer.SupervisorHeartbeat.run(SupervisorHeartbeat.java:85)
at org.apache.storm.daemon.supervisor.Supervisor.launch(Supervisor.java:202)
at org.apache.storm.daemon.supervisor.Supervisor.launchDaemon(Supervisor.java:243)
at org.apache.storm.daemon.supervisor.Supervisor.main(Supervisor.java:362)

I will attach a PR with a simple fix