Introducing bootstrapInterval for HoltWinters functions

[deniszh]

Default is '7d' - one week, as before. Format - same as for all others time intevals in Graphite.
Fixing #1957

[codecov-io]

Codecov Report

Merging #1965 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1965      +/-   ##
==========================================
+ Coverage   69.14%   69.14%   +<.01%     
==========================================
  Files          70       70              
  Lines        6954     6956       +2     
  Branches     1352     1352              
==========================================
+ Hits         4808     4810       +2     
  Misses       1912     1912              
  Partials      234      234

Impacted Files	Coverage Δ
webapp/graphite/render/functions.py	89.83% <100%> (+0.01%)	⬆️

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[DanCech]

I like it. It doesn't support passing a bare number to mean "X intervals" like movingAverage does, but adding that would make things a fair bit more complicated for little gain imho. If we want to support that later I'd want to refactor the handling for that out into a separate helper we can share with movingAverage, etc.

[deniszh]

Yes, agreed. Now all logic in HW bootstrap is based on seconds, so, decided to not put more complexity there, at least now.

[deniszh]

It's fixing the issue, so, merging.