add multiattach support in Newton

[weibt]

Several issues occured when try to use multiattach volume.
Root cause is volume status's inconsistent when try to attach/detach second VM.
Private code changes used as workaround to make volume status compatible

https://review.openstack.org/513399

Issues:
1. Attach volume with multiattach to two VMs, volume status changes to 'attaching' and get stucked;
2. Detach volume with multiattach attached to two VMs, volume status changes to 'detaching', stucked;
3. Create stack with multiattach volume failed in Heat due to volume status stuck in 'attaching';
4. Detach volume from one VM terminates iscsi connection, makes the other VM lost connection and volume not available;
   as #1471278 target removal on detaching volume with multi-attach flag

[Sean McGinnis (sean-mcginnis)]

Multiattach is being worked on and will hopefully be in Queens if the Nova work can be completed.

This is new functionality that will not be backported to Newton.