Enhance 'kernel_upgrade' across supported OSes

kernel_upgrade currently searches for the driver
"/lib/modules/${::kernelrelease}/kernel/drivers/scsi/vmw_pvscsi.ko",
which isn't reliable on RHEL/CentOS 5. I've added some conditional
logic to search for vmxnet3.ko instead, which appears to be
universally installed regardless of distro (albeit in different
locations across the different versions of RHEL/CentOS).
Merge pull request #14 from mckern/search_for_vmxnet_driver

Enhance 'kernel_upgrade' across supported OSes