Description of problem:
Every time we execute "service waagent start" it will create a daemon
process. If we execute "service waagent start" several times, there will be
more than one "python /usr/sbin/waagent -daemon" processes.
And running "service waagent stop" can only stop the latest process.

Version-Release number of selected component (if applicable):
WALinuxAgent-2.0.16-1.el6

How reproducible:
Always

Steps to Reproduce:
1. Prepare a VM with WALinuxAgent package installed.
# ps aux|grep waagent, there should be one "python /usr/sbin/waagent -daemon"
processes.
2. Excute "service waagent start".
3. Check the number of waagent daemon process.
# ps aux|grep waagent

Actual results:
There're two "python /usr/sbin/waagent -daemon" processes.

Expected results:
There's only one "python /usr/sbin/waagent -daemon" process.
No matter how many times we execute the start command, there should be only
one waagent daemon process.

Additional info:
rhel7 doesn't have this issue.