init-ceph: do umount when the path exists.

init-ceph: do umount when the path exists.

If the specified mount point is in use, umount it instead
of skipping mounting the fs.

In previous code , if we forgot that we already mount something unrelated to
*this ceph osd * to the particular mount point, we will skip the mount and
finally get an error complaining superblock not matching OSD ID.

umount & remount is better because
1. If the wrong FS not in use, we can get the right FS we want and make ceph boot smoothly.
2. If the wrong FS is in use, we will get EBUSY on umount, which seems explain the situation
more clearly than superblock mismatch.

Signed-off-by: Xiaoxi Chen <xiaoxi.chen@intel.com>