fix(coreos-base/oem-cloudstack): fix DHCP server resolution

The path for network leases has been change in recent versions
of systemd. This fix modified the DHCP server resolution script
used by CloudStack to reflect this change.
Merge pull request #792 from petrhosek/oem-cloudstack-networkd-leases-fix

fix(coreos-base/oem-cloudstack): fix DHCP server resolution