conf: Add <lease/> option for <dhcp/> settings

conf: Add <lease/> option for <dhcp/> settings

If an user is trying to configure a dhcp neetwork settings, it is not
possible to change the leasetime of a range or a host entry. This is
available using dnsmasq extra options, but they are associated with
dhcp-range or dhcp-hosts fields. This patch implements a leasetime for
range and hosts tags. They can be defined under that settings:

    <dhcp>
      <range ...>
        <lease/>
      </range>
      <host ...>
        <lease/>
      </host>
    </dhcp>

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=913446

Signed-off-by: Julio Faracco <jcfaracco@gmail.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>