mds: configurable uid/gid on new root inos

mds: configurable uid/gid on new root inos

This is to deal with the slightly perverse situation
where a user might have all the permissions they need
to create a filesystem and mount it with fuse, but then
be unable to do anything inside the mount because
we default to creating a root-owned "/" inode.

This is immediatley useful for dev/test environments without
root, but also potentially in production environments
where the filesystem configuration/mounting is done by
someone less privileged than root.

Signed-off-by: John Spray <john.spray@redhat.com>