Reduce attrgenmgr logging

[douglas-reid]

Fixes https://github.com/istio/mixer/issues/1513.

Release note:

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This change is 

[geeknoid]

/lgtm

[istio-merge-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: geeknoid

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[istio-merge-robot]

/test all [submit-queue is verifying that this PR is safe to merge]

[codecov[bot]]

Codecov Report

Merging #1514 into master will decrease coverage by 0.02%. The diff coverage is 25%.

@@            Coverage Diff             @@
##           master    #1514      +/-   ##
==========================================
- Coverage   84.57%   84.55%   -0.03%     
==========================================
  Files         122      122              
  Lines       11997    11999       +2     
==========================================
- Hits        10147    10146       -1     
- Misses       1649     1651       +2     
- Partials      201      202       +1

Impacted Files	Coverage Δ
pkg/aspect/attrgenmgr.go	91.54% <25%> (-4.11%)	:arrow_down:

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[istio-merge-robot]

Automatic merge from submit-queue.