backstore: add non-master/slave monitor to XML metadata

This silences a warning from crmsh stating that the monitor operation may not be
supported.

Signed-off-by: Chris Boot <crb@tiger-computing.co.uk>
backstore: remove some redundant comments

Signed-off-by: Chris Boot <crb@tiger-computing.co.uk>
backstore: first pass at enabling master/slave mode

This is enough to make the resource come up in master mode on one node and slave
on another, which twiddles the ALUA status for the LUN on each node. Not
extensively tested at all.

Signed-off-by: Chris Boot <crb@tiger-computing.co.uk>