Remove unused hardcoded rpc_backend from template

Glance does not yet use the openstack common oslo.messaging layer
for notify.  This option is not needed anymore, it is derived
based upon notification_strategy setting.
Updated README to reflect this.

Related-Bug: 1257748
Change-Id: I9d7709655b9680e2f435b157245c2ed58a6fe32e
Update ChefSpec to 3.4.0

This change gives us new capabilities, such as matching notifies and
subscribes of resources.

Implements: blueprint chefspec-3-4
Change-Id: Ifdba2b73801fca7f01af89e6c17d7c87bcd308e8