Security: Removing step 0 'git pull'

If anyone got access to the riptano/ComboAMI repository, they could push
a commit that would immediately start running arbitrary code on any datastax
AMI launch in the world, as long as the launched instance can talk back to
github to auto-pull the exploit.

The obvious downside is that it's harder to ship ComboAMI fixes/tweaks
because it involves building a whole new AMI for any slight change.

Other options for better security:
a) fork ComboAMI, guard your fork jealously, build your own AMI based on your fork, or
b) apply a firewall to avoid talking back to github (VPC or iptables), or
c) build your cassandra clusters without the ComboAMI automation (roll your own)

All of those other options sound like a big pain -- it would be nice to have
a more secure default launch.

If this commit is accepted, then it would be harder to run arbitrary code.
It would be necessary to both 1) own riptano's computer, and 2) insert logic
into the AMI at build time.  Or it would be necessary to gain access to the
datastax AWS account and insert the logic into the AMI itself.  Both of
which seem to me to be better guarded than access to a single git repo.

I think that the pain of having to update the AMI pays down a significant
central security risk, and it's worth the inconvenience.