Relax Kerberos login contexts parsing by trimming whitespaces in contexts list

The Kerberos login contexts list parsing right now isn't quite user-friendly.
The list must be provided as: security.kerberos.login.contexts: Client,KafkaClient, without any whitespace in between.

We can relax this to be more user-friendly by trimming any whitespaces in the list.

A user actually stumbled across this: http://apache-flink-user-mailing-list-archive.2336050.n4.nabble.com/Problems-with-Kerberos-Kafka-connection-in-version-1-2-0-td12580.html#a12589