Parameterize disk_cachemodes

The current manifests don't expose a way for end users to set
the disk_cachemodes directive in nova.conf, which can be useful
for improving disk performance.  This patch adds a parameter
allowing users to specify a list of values to be written in
to the disk_cachemodes setting.  The default value is an empty
list, which will cause the disk_cachemodes setting to be removed
entirely from nova.conf (thus matching the Nova default and
current behavior).

Change-Id: I9173ef79e4e573515f290734535907794dda7dbf
Merge "Parameterize disk_cachemodes"