backstore: dynamically calculate master score

This is now based on the number of member target ports of the ALUA target port
group. This patch prevents a backstore with no target ports (e.g. not exported
to initiators) from being a master.

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

This allows the target to use a specific ALUA target port group rather than
'default_tg_pt_gp'. Additionally, as a special case, the value is '@hostname@'
is replaced with the current node's hostname so that a different port group can
be used on different nodes with a cloned iscsi resource.

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