Add support for CMS Monitoring VictoriaMetrics back-end

[vkuznet]

I added support to send data to VictoriaMetrics, similar to ES, AMQ.

[vkuznet]

Even though I put http://cms-monitoring.cern.ch:30422 as default VM URL, for testing we should use port 30423, i.e. --vm_url=http://cms-monitoring.cern.ch:30423 option.

[cronosnull]

There is a missing import for htcondor_es.vm both in the queues.py and the history.py files. Also, the current approach is not working, it fails with "too many values to unpack" error.
The victoriametrics API expects to receive a message in a specific format, so the one created in the convert_to_json will not work.
I'm not sure if store the full documents in victoria metrics is a good approach. Instead, selecting a set of tags and metrics can work better. I think the use case for victoria metrics is closer to Influxdb than to ES, so a similar strategy should be used.

[vkuznet]

Christian, I adjusted code wrt your comments. Please review again. I made it configurable that we can pass which attributes we'd like to extract from classAds doc to send to VM. My understanding that classAds is a dict with simple key:value structure. Therefore code will allow to pass list of attributes we'd like to keep and store in VM.