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.