Remove rbd cache support

patches/ganeti/add_rbd_cache_support.patch

-Index: ganeti-3.0.1/lib/hypervisor/hv_kvm/__init__.py
-===================================================================
---- ganeti-3.0.1.orig/lib/hypervisor/hv_kvm/__init__.py
-+++ ganeti-3.0.1/lib/hypervisor/hv_kvm/__init__.py
-@@ -921,12 +921,13 @@ class KVMHypervisor(hv_base.BaseHypervis
-     disk_cache = up_hvp[constants.HV_DISK_CACHE]
-     for cfdev, link_name, uri in kvm_disks:
-       if cfdev.dev_type in constants.DTS_EXT_MIRROR:
--        if disk_cache != "none":
-+        if disk_cache != "none" and cfdev.dev_type != "rbd":
-           # TODO: make this a hard error, instead of a silent overwrite
-           logging.warning("KVM: overriding disk_cache setting '%s' with 'none'"
-                           " to prevent shared storage corruption on migration",
-                           disk_cache)
--        cache_val = ",cache=none"
-+          disk_cache = "none"
-+        cache_val = ",cache=%s" % disk_cache
-       elif disk_cache != constants.HT_CACHE_DEFAULT:
-         cache_val = ",cache=%s" % disk_cache
-       else: