Add failure for missing interface
Add failure for missing interface

This will fail with a useful error message in case the interface we are
trying to get an address for does not exist. Currently the user will
only get an unspecific "undefined method [] for nil" error.

Change-Id: I392564324fb5203df3ce390aff990ff976058320

openstack/cookbook-openstack-common,tomzo/cookbook-openstack-common,tomzo/cookbook-openstack-common,openstack/cookbook-openstack-common,tomzo/cookbook-openstack-common