isolated creadentials are not cleaned up

Due to type introduced last weak, the isolated credentials are
not claned up.
In a neutron envoriements it can mean more than 1 Gigabyte memory used
in dnsmasq and in the neutron-ns-metadata-proxy.

Change-Id: I09c538ed1139380f8cd8a662e60d69d3f605cd67
1 file changed