ceph-disk: bootstrap-osd keyring ignores --statedir

The STATEDIR variable is used to initialize the bootstrap-osd keyring
before it gets a chance to be overriden by --statedir. Replace it with
{statedir} so that it can be substituted after all options have been
parsed.

http://tracker.ceph.com/issues/9653 Fixes: #9653

Signed-off-by: Loic Dachary <loic-201408@dachary.org>