Add PowerDNS server to the list of dns servers

If dns is enabled, then add the PowerDNS server to the list of dns servers.
[deployer] Fix service name for PowerDNS when updating service ip address

Deployer updates addresses found at the microBosh spec with the assigned
microBosh address. For PowerDNS it should look for dns["address"] instead
of powerdns["address"].
[agent] guard against dns server list being nil
[agent] Print error message when failing to load infraestructure settings