vstart: set fsid in [global]

If not set, commands that rely on --show-config-value fsid or something
equivalent will fail. ceph-disk does, for instance and setting the fsid
in CEPH_ARGS won't help because it will be appended after
--show-config-value :

    CEPH_ARGS="--fsid 96a3abe6-7552-4635-a79b-f3c096ff8b95" ./ceph-mon --cluster=ceph --name=osd. --show_args --show-config-value fsid
    args: --cluster=ceph --name=osd. --show_args --show-config-value fsid --fsid 96a3abe6-7552-4635-a79b-f3c096ff8b95
    00000000-0000-0000-0000-000000000000

Signed-off-by: Loic Dachary <loic@dachary.org>