[SPARK-4269][SQL] make wait time configurable in BroadcastHashJoin

In BroadcastHashJoin, currently it is using a hard coded value (5 minutes) to wait for the execution and broadcast of the small table.
In my opinion, it should be a configurable value since broadcast may exceed 5 minutes in some case, like in a busy/congested network environment.

Author: Jacky Li <jacky.likun@huawei.com>

Closes #3133 from jackylk/timeout-config and squashes the following commits:

733ac08 [Jacky Li] add spark.sql.broadcastTimeout in SQLConf.scala
557acd4 [Jacky Li] switch to sqlContext.getConf
81a5e20 [Jacky Li] make wait time configurable in BroadcastHashJoin