Enhance 'kernel_upgrade' across supported OSes

<tt>kernel_upgrade</tt> currently searches for the driver <tt>"/lib/modules/${::kernelrelease}/kernel/drivers/scsi/vmw_pvscsi.ko"</tt>, which isn't reliable on RHEL/CentOS 5. I've added some conditional logic to search for <tt>vmxnet3.ko</tt> instead, which appears to be universally installed regardless of distro (albeit in different locations across the different versions of RHEL/CentOS).

This has been tested across the CentOS flavors mentioned but it probably needs at least a little bit more tire kicking on Ubuntu. I didn't have any clients handy tonight, so please merge with a grain of salt.