[Issue 196] feature: datasource to find VM in cluster

[castorsky]

I have implemented a datasource which was described in #196.

This datasource searches the PVE cluster for some VMs that matches specified filters and returns VM ID for one (and only one) guest. I have chosen such behavior because it will be straightforward for proxmox-clone builder - there will be no need for user to manipulate lists in locals.

I have included some tests and documentation for datasource (but since this is my very first attempt to publish my work for Packer I may have missed something).

This datasource uses undocumented meta field from Proxmox API responses. (But the Proxmox team recommended using this field to determine guest creation time in the forum messages).

Closes #196

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[mogrogan]

Thank you for your solid contribution!

I've left 2 comments but it seems pretty good.

[castorsky]

In the process of renaming the function and editing the documentation, I noticed two edge cases and fixed them

[camaeel]

@mogrogan Hi, any chances for having this merged and released?

[castorsky]

I want to mention this issue from Proxmox Bugzilla: https://bugzilla.proxmox.com/show_bug.cgi?id=6156
There is a caveat using undocumented ctime field: all machines created from template have this field set to the template's creation time, not the machine's actual time.