I thought this change was wrong because the `MESOS_REGEX` at https://github.com/apache/spark/blob/master/core/src/main/scala/org/apache/spark/SparkContext.scala#L1437 is `"""(mesos|zk)://.*""".r` but after reading through the code this change is actually correct.

I'm not sure that `mesos://zk://host:2181` is a [valid URL](https://en.wikipedia.org/wiki/Uniform_resource_locator#Syntax) though.  JDBC for example uses `jdbc:postgresql://` like in `jdbc:postgresql://localhost/test` rather than `jdbc://postgresql://...`  Issue for a later time?