[query] Fix Graphite asPercent function to handle nodes and multiple total divisor series

[robskillington]

What this PR does / why we need it:

Fixes asPercent to handle nodes arg and multiple divisor series.

Special notes for your reviewer:

Does this PR introduce a user-facing and/or backwards incompatible change?:

NONE

Does this PR require updating code package or user-facing documentation?:

NONE

[wesleyk]

LGTM

[codecov]

Codecov Report

Merging #3142 (965f566) into master (8df132f) will increase coverage by 1.6%.
The diff coverage is n/a.

❗ Current head 965f566 differs from pull request most recent head eb8cc84. Consider uploading reports for the commit eb8cc84 to get more accurate results

@@            Coverage Diff            @@
##           master    #3142     +/-   ##
=========================================
+ Coverage    70.9%    72.6%   +1.6%     
=========================================
  Files        1098     1099      +1     
  Lines      102237   103350   +1113     
=========================================
+ Hits        72586    75102   +2516     
+ Misses      24495    23138   -1357     
+ Partials     5156     5110     -46     

Flag	Coverage Δ
aggregator	76.8% <0.0%> (+<0.1%)	⬆️
cluster	84.9% <0.0%> (ø)
collector	84.3% <0.0%> (ø)
dbnode	78.9% <0.0%> (+3.3%)	⬆️
m3em	74.4% <0.0%> (ø)
m3ninx	73.5% <0.0%> (ø)
metrics	19.8% <0.0%> (ø)
msg	74.6% <0.0%> (ø)
query	68.3% <0.0%> (+1.3%)	⬆️
x	80.6% <0.0%> (+0.2%)	⬆️

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