Introducing bootstrapInterval for HoltWinters functions

[deniszh]

Backport of PR #1965 to 1.0.x

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

[deniszh]

Simple cherry-pick of my commits for #1965

[codecov-io]

Codecov Report

Merging #1977 into 1.0.x will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##            1.0.x    #1977      +/-   ##
==========================================
+ Coverage   66.28%   66.29%   +0.01%     
==========================================
  Files          56       56              
  Lines        6606     6608       +2     
  Branches     1319     1319              
==========================================
+ Hits         4379     4381       +2     
  Misses       2004     2004              
  Partials      223      223

Impacted Files	Coverage Δ
webapp/graphite/render/functions.py	89.88% <100%> (+0.01%)	⬆️

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[deniszh]

Merging