Updated schema, template to support netmask in libvirt network provisioning

[samvarankashyap]

$subject
fixes #939

Test template output:

<network ipv6='yes'>
  <name>linchpin-centos74</name>
  />
  <forward mode="nat"/>
  <domain name="linchpin.net" localOnly="yes"/>
  <ip address="192.168.74.100"  netmask="255.255.255.0" >
    <dhcp>
      <range start="192.168.74.101" end="192.168.74.112"/>
    </dhcp>
  </ip>
</network>

[samvarankashyap]

@p3ck Will try to add test to this PR. I am not sure whether it would pass though.

[adl-bot]

ci-linchpin-messageBus-trigger build #1208: SUCCESS: https://jenkins-continuous-infra.cloud.continuous-infra.upshift.redhat.com/job/ci-linchpin-messageBus-trigger/1208/

[samvarankashyap]

[merge]