Upgrade CentOS integration test AMI to 5.8

Use another RightScale AMI.
Add integration test for CentOS 6.3

Use official CentOS 6 AMI:
  http://wiki.centos.org/Cloud/AWS

The image has iptables firewall enabled that only allow SSH connections
in and thus makes the Apache cookbook test to fail. So we just shut down
the whole firewall.
Merge pull request #237 from tmatilai/centos-integrations-tests

CentOS 5.6 integration test fails