Add instance flavor monitoring

Description

Add new metric flavor to "telegraf-libvirt-metrics".

Proposal

Use "nova:flavor" node from libvirt:

<metadata>
    <nova:instance xmlns:nova="http://openstack.org/xmlns/libvirt/nova/1.0">
      <nova:package version="13.1.1"/>
      <nova:name>test-to-migrate</nova:name>
      <nova:creationTime>2019-05-21 06:32:35</nova:creationTime>
      <nova:flavor name="32CPU-80RAM-320HDD">
        <nova:memory>81920</nova:memory>
        <nova:disk>320</nova:disk>
        <nova:swap>0</nova:swap>
        <nova:ephemeral>0</nova:ephemeral>
        <nova:vcpus>32</nova:vcpus>
      </nova:flavor>
      <nova:owner>
        <nova:user uuid="8cde5603e78f44e8ade3ed3e5685dc10">porzech</nova:user>
        <nova:project uuid="1194c60e950b471eaefaa6329469e2fd">test</nova:project>
      </nova:owner>
      <nova:root type="image" uuid="9c7e776b-f6c7-422c-8cb3-08de72383c1d"/>
    </nova:instance>
  </metadata>

Links / references