Label->tag mapper: produce InventoryCollection<Tag>

[cben]

This works same as last PR #16098 for save_inventory mode but also exposes a more concrete interface that I'll be able to use in graph refresh:

• Creation of missing tags: The tags_to_resolve_collection can just be added to graph refresh.

• Reassigning tags: each map_labels returns individual InventoryObject - those can used to build
  InventoryCollection that will work similar to retag_entity.

  (We originally planned to do reassignment using original code in post-refresh.
  I'm postponing the move to post-refresh ;-) because we it was motivated by a new feature (going through policy) that is less critical than getting tagging working as now;
  and IIUC I'd need to express desired taggings in some form for post-refresh anyway.

(PRs for both of the above that upcoming, WIP building right query similar to retag_entity and adding tests...)

Overview issue: ManageIQ/manageiq-providers-kubernetes#126

@miq-bot add-label enhancement

@Ladas @zgalor @moolitayer @djberg96 please review.

[cben]

hmm, need to debug these failures.
@miq-bot add-label wip

[miq-bot]

Checked commit cben@09986c0 with ruby 2.3.3, rubocop 0.47.1, and haml-lint 0.20.0
5 files checked, 0 offenses detected
Everything looks fine. ⭐

[cben]

@miq-bot remove-label wip

@Ladas @agrare please review.

The specific_tags_collection is silly bussiness. I just need something with known .id I can refer to
from an InventoryCollection<Tagging> (these will appear in next k8s and openshift PR based on this one). Actually it's dead code, UI doesn't allow (and never allowed) creating specific-value -> specific-tag mappings, I'm going to drop them, just requires rewriting tons of tests, so not in this PR...