Use corosync.nodes when creating list of nodes in corosync cluster

List of nodes is specified in 'corosync' context. This allows
more fine-grained customization of corosync list of nodes independently
on list of controllers nodes.

Neither 'controllers' nor 'corosync.nodes' are currently defined in
our heat templates.

Change-Id: I086a31c24500d372f86412e6e349be7797929ddd
Merge "Use corosync.nodes when creating list of nodes in corosync cluster"