Security: Removing step 0 'git pull'

If anyone contrives to gain access to the riptano/ComboAMI repository, they
could push a commit that for any subsequent datastax-ami-launched instance,
if that instance could talk back to github to do the pull, arbitrary code
could be run.  They would be able to do this without gaining access to the
datastax AWS account.  They would then have access inside the AWS account
that cassandra is being launched in.

Other options for better security:
a) fork ComboAMI, apply this patch, 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 pretty inconvenient -- 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 (or whoever build
the AMI), 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 post-build.  Both of which would seem to me to be less likely
than gaining access to a single git repo.

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.

I think that the pain of having to update the AMI more frequently pays down
a significant central security risk, and it's worth the inconvenience.
Revert "Security: Removing step 0 'git pull'"

This reverts commit 117f96187e8647c46a68a55f4f2b38041665ae97.

This is impractical because building/replacing AMIs for all the regions is
so inconvenient for both the datastax developers, and for consumers who rely
on the image id being stable.

Perhaps a better idea would be to pass a commit SHA1 in as an optional
parameter and just validate it after the first 'git pull', for those who
care.