Add spec for 'Failover to alternative iSCSI portals on login failure'

When the main iSCSI portal is unreachable by network failure etc.,
volume attach/detach will fail even though the other portal addresses
is reachable. To enable nova-compute to fall-back to alternative portal
addresses, cinder should tell the alternative portal addresses to nova.

Change-Id: Ie19c1c8bb66c30e3438708951607c9bb1005839d
blueprint: iscsi-alternative-portal
Merge "Add spec for 'Failover to alternative iSCSI portals on login failure'"