[SPARK-11859][MESOS] SparkContext accepts invalid Master URLs in the form...

[SPARK-11859][MESOS] SparkContext accepts invalid Master URLs in the form zk://host:port for a multi-master Mesos cluster using ZooKeeper

* According to below doc and validation logic in [SparkSubmit.scala](https://github.com/apache/spark/blob/master/core/src/main/scala/org/apache/spark/deploy/SparkSubmit.scala#L231), master URL for a mesos cluster should always start with `mesos://`

http://spark.apache.org/docs/latest/running-on-mesos.html
`The Master URLs for Mesos are in the form mesos://host:5050 for a single-master Mesos cluster, or mesos://zk://host:2181 for a multi-master Mesos cluster using ZooKeeper.`

* However, [SparkContext.scala](https://github.com/apache/spark/blob/master/core/src/main/scala/org/apache/spark/SparkContext.scala#L2749) fails the validation and can receive master URL in the form `zk://host:port`

* For the master URLs in the form `zk:host:port`, the valid form should be `mesos://zk://host:port`

* This PR restrict the validation in `SparkContext.scala`, and now only mesos master URLs prefixed with `mesos://` can be accepted.

* This PR also updated corresponding unit test.

Author: toddwan <tawan0109@outlook.com>

Closes #9886 from toddwan/S11859.