Added t2 instance types

New t2 instance types require HVM amis, bailout assumption of pvm
causes failures when using t2 instance types.

Author: Basit Mustafa <basitmustafa@computes-things-for-basit.local>

Closes #1446 from 24601/master and squashes the following commits:

01fe128 [Basit Mustafa] Makin' it pretty
392a95e [Basit Mustafa] Added t2 instance types
Reservoir sampling implementation.

This is going to be used in https://issues.apache.org/jira/browse/SPARK-2568

Author: Reynold Xin <rxin@apache.org>

Closes #1478 from rxin/reservoirSample and squashes the following commits:

17bcbf3 [Reynold Xin] Added seed.
badf20d [Reynold Xin] Renamed the method.
6940010 [Reynold Xin] Reservoir sampling implementation.