Reducing the erroneous download events to send only periodically

[vrdmr]

Description

Also, activating the skipped unittests.

Issue #

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PR information

• The title of the PR is clear and informative.
• There are a small number of commits, each of which has an informative message. This means that previously merged commits do not appear in the history of the PR. For information on cleaning up the commits in your pull request, see this page.
• Except for special cases involving multiple contributors, the PR is started from a fork of the main repository, not a branch.
• If applicable, the PR references the bug/issue that it fixes in the description.
• New Unit tests were added for the changes made and Travis.CI is passing.

Quality of Code and Contribution Guidelines

• I have read the contribution guidelines.

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This change is 

[codecov]

Codecov Report

Merging #1567 into develop will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           develop   #1567      +/-   ##
==========================================
+ Coverage    63.37%   63.4%   +0.03%     
==========================================
  Files           78      78              
  Lines        11058   11063       +5     
  Branches      1559    1561       +2     
==========================================
+ Hits          7008    7015       +7     
+ Misses        3711    3709       -2     
  Partials       339     339

Impacted Files	Coverage Δ
azurelinuxagent/ga/exthandlers.py	81.61% <100%> (+0.32%)	⬆️

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[codecov]

Codecov Report

Merging #1567 into develop will increase coverage by 0.05%.
The diff coverage is 84.21%.

@@             Coverage Diff             @@
##           develop    #1567      +/-   ##
===========================================
+ Coverage    63.37%   63.43%   +0.05%     
===========================================
  Files           78       78              
  Lines        11058    11070      +12     
  Branches      1559     1562       +3     
===========================================
+ Hits          7008     7022      +14     
+ Misses        3711     3709       -2     
  Partials       339      339

Impacted Files	Coverage Δ
azurelinuxagent/common/protocol/wire.py	69.2% <50%> (ø)	⬆️
azurelinuxagent/ga/exthandlers.py	81.75% <88.23%> (+0.46%)	⬆️
azurelinuxagent/common/cgroupapi.py	62.5% <0%> (ø)	⬆️

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[pgombar]

LGTM