Function for joining Cinder backend arrays

When a cinder backend was disabled, the corresponding variable
(e.g. $e_backend) didn't get defined and Puppet would print this
error:

Error: Could not retrieve catalog from remote server: Error 400 on
SERVER: Failed to parse inline template: Could not find value for
'e_backend' at line 1 at
/usr/share/astapor/puppet/modules/quickstack/manifests/cinder_controller.pp:188
on node r64-acontrol.example.org

Implement a function that can cope with undefined variables.