ceph-disk-activate: specify full path for blkid

/sbin apparently isn't in the path when udev runs us.

Signed-off-by: Sage Weil <sage@inktank.com>
ceph-disk-activate: auto detect init system

Look for an option 'init' in ceph.conf.  Otherwise, check if we're ubuntu.
If so, use upstart.  Otherwise, use sysvinit.

Signed-off-by: Sage Weil <sage@inktank.com>
udev: trigger ceph-disk-activate directly from udev

There is no need to depend on upstart for this.

Signed-off-by: Sage Weil <sage@inktank.com>