fence_sbd: Add a fencing agent for sbd

Add a STONITH agent for SDB (STORAGE BASED DEATH).

Signed-off-by: Adrian Vondendriesch <adrian.vondendriesch@credativ.de>

Add option --sbd-path & XML metadata

Remove magic numbers

--delay should work as expected

one more magic number

Use "method" from fencing library

fence_sdb: rename parameter --sbd-devices

Rename --sbd-devices to --devices. Other agents have identical parameters and
call them "--devices". So keep this parameter equal over the different fencing
agents.

fence_sbd: Update fence_sdb xml data.

Update fence_sbd xml meta data to catch up with the current state.