ocfs2/cluster: Get all heartbeat regions

Export function in o2hb to get a list of heartbeat regions. It also adds an
upper limit to the length of the heartbeat region name.

o2hb_global_heartbeat_active() currently disables global heartbeat. It will
be enabled in a later patch after all the code is added.

Signed-off-by: Sunil Mushran <sunil.mushran@oracle.com>