Removed unnecessary EL7 commands and optimized CW agent download script

[eemperor]

The default CloudWatch agent config script, amazon-cloudwatch-agent-ctl, already handles setting up the CloudWatch agent service in EL6 and EL7. The two systemd commands were not required.

In additon, the CloudWatch agent download steps have been optimized into fewer commands.

This PR addresses the following issues:

#15
#16

[ferricoxide]

@eemperor Quick query: did you validate that the CWAgent works as expected on EL6? That it comes with a systemd unit-file but not a legacy-init file is somewhat concerning?

[eemperor]

One of my tests was EL6 on CentOS but not RedHat. Let me double check.

[ferricoxide]

Cool. CentOS should be adequately representative

Did the CWA processes come back online after a reboot of the CentOS 6.9 instance? If so, did the installer create /etc/rc.d files or is it using a different mechanism to ensure the service restarts across boots?

[eemperor]

The CWA process does come back running after reboot following completion of the Cfn stack. I will have to do additional testing to speak on the resilincy across multiple reboots.

It appears the CWA installer populates both a .conf file in /etc/init and a .service in /etc/systemd/system/ then executes one or the other based on the system. So looks like it's using Upstart to launch the CWA process in EL6?

[ferricoxide]

Good to know: I hadn't noticed the .conf file when I was looking at the RPM.

But, yeah, the .conf file means they're using upstart rather than chkconfig for enabling persistency.

[lorengordon]

🚢