vstart.sh: only skip fs volume create when there's no mgr to run it

vstart.sh: only skip fs volume create when there's no mgr to run it

The mgr volumes module handles fs volume ls/create, not the monitor, so
without a mgr (CEPH_NUM_MGR=0) the "wait for volume module to load" loop
spun forever. Open-coding fs new unconditionally, my first attempt,
changed pool creation for every vstart user and dropped the wait, so
cephfs tests calling `fs volume ...` right after vstart.sh returns could
race a module still mid-load.

create_fs_volume() branches on CEPH_NUM_MGR instead. With a mgr it still
calls fs volume ls/create. Without one it runs the plain mon commands fs
volume create wraps: osd pool create cephfs.<name>.{meta,data}, then fs
new <name> <meta> <data>. The wait stays at its one call site above the
per-filesystem loop, so it runs once regardless of CEPH_NUM_FS, not once
per filesystem.

Verified against a local vstart: mgr present, fs volume ls polls once
then creates CEPH_NUM_FS=2 filesystems with no repeated poll;
CEPH_NUM_MGR=0, no poll, fs new creates 'a' directly, HEALTH_OK.

Signed-off-by: Kefu Chai <k.chai@proxmox.com>