updating default bigtop version to 0.8.0

updating default bigtop version.  verified in vagrant.

```
hadoop-hdfs-2.4.1.6-1.el6.x86_64
hadoop-hdfs-secondarynamenode-2.4.1.6-1.el6.x86_64
hadoop-2.4.1.6-1.el6.x86_64
hadoop-client-2.4.1.6-1.el6.x86_64
hadoop-yarn-resourcemanager-2.4.1.6-1.el6.x86_64
hadoop-yarn-2.4.1.6-1.el6.x86_64
hadoop-hdfs-namenode-2.4.1.6-1.el6.x86_64
hadoop-yarn-nodemanager-2.4.1.6-1.el6.x86_64
hadoop-mapreduce-2.4.1.6-1.el6.x86_64
hadoop-hdfs-datanode-2.4.1.6-1.el6.x86_64
```