cache: Override disk_cache hvp with disk param in EXT
cache: Override disk_cache hvp with disk param in EXT

This is a hack that brings disk_cache hypervisor param
at disk level. This is supported only for disks of EXT
template that allow arbitrary params. The corresponding
param that should be used is 'cache'.

Signed-off-by: Dimitris Aragiorgis <dimitris.aragiorgis@gmail.com>

Conflicts:
	lib/hypervisor/hv_kvm/__init__.py

yiannist/ganeti,yiannist/ganeti,yiannist/ganeti