Set nova disk cachemodes to file directsync

Set nova disk cachemodes to file directsync

Set nova/libvirt disk cache modes to file directsync to bypass the
host page cache & prevent instance's kernel panic due to absent
/dev partition. Caching mode directsync effectively turns all guest
I/O operations into direct I/O operations on the host, which is the
NFS client (virtuals) or GlusterFS (baremetals).
Also return back to hwe kernel as a golden mean between GA & Edge ones.

Change-Id: I51ab7d0ee71c214ff16b756cfee16e918738b6fd
Signed-off-by: Michael Polenchuk <mpolenchuk@mirantis.com>