Forbid empty ClientID and use 'sarama' as default

[slaunay]

Documentation states that "sarama" is the default ClientID but the empty string actually is used.

This is a not a problem for connecting to the brokers but it creates issues when settings policies against the tools binaries (e.g. kafka-console-producer) like:

bin/kafka-configs.sh  --zookeeper localhost:2181 --describe --entity-name ""  --entity-type clients
Error while executing topic command Path must not end with / character
java.lang.IllegalArgumentException: Path must not end with / character
    at org.apache.zookeeper.common.PathUtils.validatePath(PathUtils.java:58)
    at org.apache.zookeeper.ZooKeeper.getData(ZooKeeper.java:1137)
    at org.apache.zookeeper.ZooKeeper.getData(ZooKeeper.java:1184)
    at org.I0Itec.zkclient.ZkConnection.readData(ZkConnection.java:119)
    at org.I0Itec.zkclient.ZkClient$12.call(ZkClient.java:1094)
    at org.I0Itec.zkclient.ZkClient$12.call(ZkClient.java:1090)
    at org.I0Itec.zkclient.ZkClient.retryUntilConnected(ZkClient.java:985)
    at org.I0Itec.zkclient.ZkClient.readData(ZkClient.java:1090)
    at org.I0Itec.zkclient.ZkClient.readData(ZkClient.java:1085)
    at org.I0Itec.zkclient.ZkClient.readData(ZkClient.java:1074)
    at kafka.admin.AdminUtils$.fetchEntityConfig(AdminUtils.scala:353)
    at kafka.admin.ConfigCommand$$anonfun$describeConfig$1.apply(ConfigCommand.scala:108)
    at kafka.admin.ConfigCommand$$anonfun$describeConfig$1.apply(ConfigCommand.scala:107)
    at scala.collection.immutable.List.foreach(List.scala:318)
    at kafka.admin.ConfigCommand$.describeConfig(ConfigCommand.scala:107)
    at kafka.admin.ConfigCommand$.main(ConfigCommand.scala:57)
    at kafka.admin.ConfigCommand.main(ConfigCommand.scala)

Fix uses "sarama" as the default ClientID following the documentation and also forbids the empty string (test case provided).

[eapache]

There's no reason to make this a public constant.

Also, while I understand the intent behind putting it in client.go I think it makes more sense in config.go.

[eapache]

Wow I'm dumb. I had two distinct chances to get this trivial thing right: when I wrote the original PR that said "it defaults to sarama!" and a whole year later when I wrote the regex and had a chance to use the correct quantifier, but didn't.

Thanks for finding this :)

[slaunay]

I applied the changes.
Let me know if you want me to rebase the changes.

Thanks for this sweet library, much easier to read than the official Java client.

[eapache]

Thanks :)