Update nova_rpc_backend to correct setting
Update nova_rpc_backend to correct setting

The nova_rpc_backend variable is currently set to the value that
applies for Kilo and has not been updated to the new Liberty
setting.

This patch corrects that.

Closes-Bug: #1537039
Change-Id: I6a946a5ce56a90e9ef1aebdba0350076cdb9ad2c

stackforge/os-ansible-deployment,mrda/openstack-ansible,openstack/openstack-ansible,robb-romans/openstack-ansible,mancdaz/openstack-ansible,major/openstack-ansible,robb-romans/openstack-ansible,yanyao/newton,BjoernT/os-ansible-deployment,mrda/openstack-ansible,bunchc/openstack-ansible,BjoernT/os-ansible-deployment,major/openstack-ansible,BjoernT/openstack-ansible,os-cloud/os-ansible-deployment,cfarquhar/openstack-ansible,weezer/openstack-ansible,mancdaz/openstack-ansible,bunchc/openstack-ansible,cloudnull/os-ansible-deployment,os-cloud/os-ansible-deployment,major/os-ansible-deployment,cloudnull/os-ansible-deployment,mrda/openstack-ansible,openstack/openstack-ansible,yanyao/openstack-ansible,Logan2211/openstack-ansible,yanyao/openstack-ansible,stackforge/os-ansible-deployment,Logan2211/openstack-ansible,weezer/openstack-ansible,major/os-ansible-deployment,BjoernT/openstack-ansible,BjoernT/os-ansible-deployment,yanyao/newton,cfarquhar/openstack-ansible