Change vhd overhead from a static default 10 GB to a nova.conf configurable

I'm changing the vhd fudge factor from a static 10 gb to allow the overhead to be modified by the openstack user. This variable will be able to be configured via nova.conf and will make sense to be used in environments that utilize the xenserver fast copy feature where the base copy depending on it's size, could potentially cause issues with the customers next build from that image. The default of this variable will remain at 10 gb, however now it will be configurable. The user will not have to add it to their conf file and if they don't the 10 gb default will be used.