Add VSphere input plugin

[mlabouardy]

Required for all PRs:

• CHANGELOG.md updated (we recommend not updating this until the PR has been approved by a maintainer)
• Sign CLA (if not already signed)
• README.md updated (if adding a new plugin)

[mkuzmin]

This PR is a good start, but we need a bit more work to reach acceptable level of quality.

I also developed pretty the same plugin, but not going to publish alternate PR. Instead, I rebased my fixes on top of your code at https://github.com/mkuzmin/telegraf/commits/vsphere.
There was a sad story of unhealthy competition in vSphere plugins for Terraform and Packer, and I really don't want to repeat the same mistakes here. Let's join the forces, and develop the best solution together.

I'd suggest the next plan:

1. You mark this PR as [WIP]
2. You rebase the code to release-1.3 branch until we finish the work. I want to use this code in production ASAP, but it should not involve changes in Telegraf core.
   Squashing commits would be helpful.
3. I will post my contributions as PRs to your repository. We discuss and agree on all the changes. I'd like to have contributor permissions, so after review I'm able to merge PRs by myself.
4. We publish binary releases and involve people to test our code and provide early feedback.

[danielnelson]

@mkuzmin @mlabouardy Have you made any progress on unifying your plugins?

[mlabouardy]

@mkuzmin I rebased the code to https://github.com/mlabouardy/telegraf/tree/release-1.3

I added you as contributor

[mkuzmin]

Thank you. I will return to this PR soon.

[huyujie]

@mlabouardy @mkuzmin when I use this vsphere plugin,output:"default datacenter resolves to multiple instances, please specify"

[ideologysec]

@huyujie export GOVC_DATACENTER="Your Datacenter Name Here"

[danielnelson]

Closing in favor of #4141