Up the /boot direcotry size to 150MB.  The current size isn't
sufficient to update the packages in /boot directory.
This is needed for Packstack and Openstack work.
Up it to 512MB to give some additional space
Merge pull request #261 from galthaus/pending/centos-boot-update

Up the /boot direcotry size to 150MB for centos/fedora/RHEL