Fix issue regarding --no-lvm-storage

If cluster is initialized with --no-lvm-storage then volume_group_name
does not exist in config.data. Thus we must define it as optional in
confd.

Hail expects interger values for vg_size and vg_free. Currently
_GetVgInfo() returns None in case of --no-lvm-storage. A workaround
is to return 0 instead.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>