[HOTFIX][STREAMING] Allow the JVM/Netty to decide which port to bind to in Flume Polling Tests.

Author: Hari Shreedharan <harishreedharan@gmail.com>

Closes #1820 from harishreedharan/use-free-ports and squashes the following commits:

b939067 [Hari Shreedharan] Remove unused import.
67856a8 [Hari Shreedharan] Remove findFreePort.
0ea51d1 [Hari Shreedharan] Make some changes to getPort to use map on the serverOpt.
1fb0283 [Hari Shreedharan] Merge branch 'master' of https://github.com/apache/spark into use-free-ports
b351651 [Hari Shreedharan] Allow Netty to choose port, and query it to decide the port to bind to. Leaving findFreePort as is, if other tests want to use it at some point.
e6c9620 [Hari Shreedharan] Making sure the second sink uses the correct port.
11c340d [Hari Shreedharan] Add info about race condition to scaladoc.
e89d135 [Hari Shreedharan] Adding Scaladoc.
6013bb0 [Hari Shreedharan] [STREAMING] Find free ports to use before attempting to create Flume Sink in Flume Polling Suite