Fix requiretty issue with devstack, reduce distros to CentOS 6.5

The stack.sh script gets run, but there's another weird issue to work out.  SSH is not responsive once the stack is in CREATE_COMPLETE.

The following is from /var/log/cloud-init-output.log.  See the part about ssh-authkey-fingerprints failing. 

```
2014-03-13 21:12:17,330 - stages.py[WARNING]: Module ca-certs is verified on ['debian', 'ubuntu'] distros but not on rhel distro. It may or may not work correctly.
Cloud-init v. 0.7.5 running 'modules:config' at Thu, 13 Mar 2014 21:12:17 +0000. Up 39.41 seconds.
2014-03-13 21:12:18,199 - stages.py[WARNING]: Module emit_upstart is verified on ['debian', 'ubuntu'] distros but not on rhel distro. It may or may not work correctly.
2014-03-13 21:12:18,217 - stages.py[WARNING]: Module ssh-import-id is verified on ['ubuntu'] distros but not on rhel distro. It may or may not work correctly.
2014-03-13 21:12:18,219 - stages.py[WARNING]: Module grub-dpkg is verified on ['debian', 'ubuntu'] distros but not on rhel distro. It may or may not work correctly.
2014-03-13 21:12:18,220 - stages.py[WARNING]: Module apt-pipelining is verified on ['debian', 'ubuntu'] distros but not on rhel distro. It may or may not work correctly.
2014-03-13 21:12:18,221 - stages.py[WARNING]: Module apt-configure is verified on ['debian', 'ubuntu'] distros but not on rhel distro. It may or may not work correctly.
2014-03-13 21:12:18,223 - stages.py[WARNING]: Module landscape is verified on ['ubuntu'] distros but not on rhel distro. It may or may not work correctly.
2014-03-13 21:12:18,228 - stages.py[WARNING]: Module byobu is verified on ['debian', 'ubuntu'] distros but not on rhel distro. It may or may not work correctly.
Cloud-init v. 0.7.5 running 'modules:final' at Thu, 13 Mar 2014 21:12:18 +0000. Up 40.31 seconds.
2014-03-13 21:12:19,078 - util.py[WARNING]: Running ssh-authkey-fingerprints (<module 'cloudinit.config.cc_ssh_authkey_fingerprints' from '/usr/lib/python2.6/site-packages/cloudinit/config/cc_ssh_authkey_fingerprints.pyc'>) failed
Cloud-init v. 0.7.5 finished at Thu, 13 Mar 2014 21:12:19 +0000. Datasource DataSourceConfigDriveNet [net,ver=2][source=/dev/xvdd].  Up 41.04 seconds
```

Seems to be a bug in cloud-init:
https://bugs.launchpad.net/ubuntu/+source/cloud-init/+bug/1073204