[Proposal] Add light-weight eagle examples without depending on Hortonworks Sandbox

[Hao Chen <ha...@apache.org>]

*Problem*

Currently when users want to have a quick try with Eagle, they have to
everything manually:

   1. Download *eagle* package
   2. Install *hortonworks sandbox *(8GB +) with *VirtualBox*
   3. Start sandbox and start *kafka, storm, hbase, hdfs *and so on (where
   the hbase is extremely unstable)
   4. Create kafka topic and Install *hdfs audit plugin,* and flow log into
   kafka
   5. Start *eagle service*, init *metadata* and start *eagle topology*
   6. Define policies and play with alert (in fact no sample data is
   provided, user have to produce their own, which is hard especially when is
   just a new user)

Above guideline is so complex that may sometimes guide user FROM START TO
GIVE-UP instead of adopting eagle :-)

*Proposed Solution*

User only need to:

   1. git clone https://github.com/apache/incubator-eagle.git
   2. *./eagle-example/hdfs-security-example*/play.sh
   - check whether source code is built, if not, execute maven to build, if
      maven is not installed, install maven firstly.
      - start local zookeeper/kafka
      - start eagle service with embedded derby local database
      - import required metadata  through local eagle service
      - start local topology
      - import sample policy
   3. *./eagle-example/hdfs-security-example*/trigger.sh
   - Send sample events which will trigger alerts from above policies

In future, we may add more use cases oriented examples:

   - ./eagle-example/cassandra-security-example/[play.sh, trigger.sh]
   - ./eagle-example/hadoop-jmx-example/[play.sh, trigger.sh]
   - ./eagle-example/mapr-securiy-example/[play.sh, trigger.sh]

Any opinions?

- Hao