fix for error in public-keys url

The URL used to pull public SSH keys is wrong for Cloudstack. Corrected to the appropriate URL which is: `http://<DHCP SERVER>/latest/public-keys`

Tested in production environment on exoscale public cloud.

NOTE: here are all endpoints available in CloudStack:

    $ curl --fail -s http://<DHCP SERVER>/latest/
    user-data
    meta-data
    service-offering
    availability-zone
    local-ipv4
    local-hostname
    public-ipv4
    public-hostname
    instance-id
    vm-id
    public-keys
    cloud-identifier