Allow multiple flume instances

It was not possible to use the script to start multiple instance. These changes allows multiple instances to be started be setting FLUME_AGENT_NAME or by writing a simple wrapper like:

---
#!/bin/bash

export FLUME_AGENT_NAME=$1 
COMMAND=$2

/etc/init.d/flume-ng-agent $COMMAND
---

Ex:

# flume-wrapper.sh start agent1
# flume-wrapper.sh start agent2

# flume-wrapper.sh stop agent1
# flume-wrapper.sh stop agent2