Add CentOS stemcell building and BATs

Signed-off-by: Andrea Law <alaw@pivotallabs.com>
Add CentOS stemcells to publish-stemcells.sh

Signed-off-by: Andrea Law <alaw@pivotallabs.com>
Update fetch_latest_bosh()

* git reset --hard instead of checkout to blow away any changes to the
  checked out bosh dir

Signed-off-by: Andrea Law <alaw@pivotallabs.com>
wip

Signed-off-by: Andrea Law <alaw@pivotallabs.com>