Add OpenStack Senlin Cluster target plugin

[dkt26111]

This adds a target plugin for OpenStack to scale Nomad cluster clients running as VMs in OpenStack using Senlin clusters.

Senlin is the OpenStack service that allows users to create clusters of VMs for autoscaling purposes. It is similar to AWS ASG.

[hashicorp-cla]

All committers have signed the CLA.

[cgbaker]

Hi @dkt26111, thank you for the contribution. We're currently in the middle of a discussion about which plugins to include into this repo and which plugins to leave for external plugins. We will get back to you ASAP regarding a decision on this; if you have any opinion on that, we'd be glad to hear your perspective.

[dkt26111]

I have considered making this as an external plugin so I'm open to that.
Is there a clean way to use scaleutils.ScaleInReq with a custom RemoteProvider in an external plugin? That seems to be the only blocker for me to make this an external plugin.

[cgbaker]

Is there a clean way to use scaleutils.ScaleInReq with a custom RemoteProvider in an external plugin? That seems to be the only blocker for me to make this an external plugin.

well-noted, @dkt26111 . we had observed that ourselves and were in the process of addressing it. i'll post an issue to track that and list it here; this will be handled in the next release (0.3.0), along with a change to use gRPC for the plugin system.

[dkt26111]

Awesome, thanks. Once that is available, I'll move this to an external plugin.

[jrasell]

Hi @dkt26111, the refactor is now available and has been released. If you come across any issues, or have any thoughts please let us know. :)

[dkt26111]

Hi @dkt26111, the refactor is now available and has been released. If you come across any issues, or have any thoughts please let us know. :)

Thanks. I will integrate the changes in the next couple days.

[dkt26111]

I tested my external plugin implementation with the nomad autoscaler 0.3.0 and verified that it works flawlessly. Thank you for your help. I will go ahead and close this PR.

For reference, the OpenStack Senlin target plugin is available here: https://github.com/dkt26111/nomad-senlin-autoscaler

[cgbaker]

hey @dkt26111 , great stuff! ❤️
we'll get this added to the list of community plugins as soon as we merge hashicorp/nomad#10125