Reduced lvm metadata size to (actual size - 8M)

The reason for that is to make sure that
there will be enough space for creating logical volumes.
Default lvm extension size is 4M. Nailgun volume
manager does not care of it and if physical volume size
is 4M * N + 3M and lvm metadata size is 4M * L then only
4M * (N-L) + 3M of space will be available for
creating logical extensions. So only 4M * (N-L) of space
will be available for logical volumes, while nailgun
volume manager might reguire 4M * (N-L) + 3M
logical volume. Besides, parted aligns partitions
according to its own algorithm and actual partition might
be a bit smaller than integer number of mebibytes.

Implements: blueprint image-based-provisioning
Change-Id: I14e5e8e37a52685a67a99fece630b368e6064c76
Merge "Reduced lvm metadata size to (actual size - 8M)"