update cloud-init for rhel

* add only useful modules per distrib
 - *dpkg for debian
 - *yum for rhel
sure, we don't need to have specific debian cloud_config_modules for rhel, it's the same for debian.

Without this change, I have this error on rhel system:

<pre>
Nov 14 15:42:58 install-server cloud-init: 2014-11-14 15:42:58,115 - stages.py[WARNING]: Could not find module named cc_apt_update_upgrade
Nov 14 15:42:58 install-server cloud-init: 2014-11-14 15:42:58,228 - util.py[WARNING]: Running locale (<module 'cloudinit.config.cc_locale' from '/usr/lib/python2.7/site-packages/cloudinit/config/cc_locale.pyc'>) failed
Nov 14 15:42:58 install-server cloud-init: 2014-11-14 15:42:58,268 - util.py[WARNING]: Failed to run debconf-set-selections for grub-dpkg
Nov 14 15:42:59 install-server cloud-init: Cloud-init v. 0.7.5 running 'modules:final' at Fri, 14 Nov 2014 15:42:59 +0000. Up 11.38 seconds.
</pre>