User can now choose to have a filesystem type different from
default ext4 by using:
export BOOTSTRAP_OPTS="bootstrap_host_data_disk_fs_type=xfs"
for instance.
This may be usefull to get closer to new distributions defaults
(like Centos) which moved to xfs. Furthermore, xfs offers
several operational advantages over ext4.
Change-Id: I5a0db09b842b74aae0aec14415c5c07a9c594705