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

libraries/network.rb

@@ -37,6 +37,7 @@ module ::Openstack
         return '0.0.0.0'
       end
     end
+    fail "Interface #{interface} does not exist" unless nodeish['network']['interfaces'][interface]
     addresses = nodeish['network']['interfaces'][interface]['addresses']
     fail "Interface #{interface} has no addresses assigned" if addresses.to_a.empty?
     get_address addresses, family, drop_vips