Enable stripped LVs

This patch enables stripped LVs, falling back to non-stripped if the
stripped creation fails. If the configure-time lvm-stripecount is 1,
this patch becomes a noop (with an insignificant python-level overhead,
but no extra lvm calls).

The effect of this patch is that new instances will get stripped LVs
from the start, whereas old instances will have their LVs stripped as
soon as replace-disks is run for them.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>