Build VimPerformanceTagValues for utilization charts by tags on the fly

[gtanzillo]

This PR is the first step towards getting rid of the vim_performance_tag_values table. The table allows for grouping metrics by assigned tags at the timestamp of each metric. This table can get huge with only a few tag categories enabled for rollup. See referenced BZs below.

After digging through the code that uses that table Nick and I discovered that it's not actually used. The table is queried but the contents are not used in the resulting chart. Instead, instances of VimPerformanceTagValue are generate on the fly.

The first commit eliminates queries to the table for report generation. It will still read and write to the table during metrics processing. Will want to remove that too.

The second commit cleans up some of the code to hopefully make it easier to follow.

/cc @carbonin @Fryguy @blomquisg

https://bugzilla.redhat.com/show_bug.cgi?id=1510484
https://bugzilla.redhat.com/show_bug.cgi?id=1514505

[miq-bot]

Checked commits gtanzillo/manageiq@c4b9ed4~...ebf649e with ruby 2.3.3, rubocop 0.47.1, haml-lint 0.20.0, and yamllint 1.10.0
2 files checked, 1 offense detected

app/models/vim_performance_tag_value.rb

• ❗ - Line 120, Col 77 - Style/EachWithObject - Use each_with_object instead of inject.

[Fryguy]

This is gaprindashvili/yes because #16692 needs to be gaprindashvili/yes and that one is dependent on this.

[simaishi]

Gaprindashvili backport details:

$ git log -1
commit 253a0125507a82124f52179362ae79bdfc08c88f
Author: Jason Frey <fryguy9@gmail.com>
Date:   Tue Jan 2 13:44:30 2018 -0500

    Merge pull request #16582 from gtanzillo/tag-values
    
    Build VimPerformanceTagValues for utilization charts by tags on the fly
    (cherry picked from commit bd2fa2fca0d419795e437aa44e1c76af128a97c1)
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1512705