Use Cloudfront instead of direct-to-S3

[trishankatdatadog]

What does this PR do?

Point our downloader to our Cloudfront distribution instead of our S3 bucket directly.

Motivation

Some customers have blacklisted amazonaws.com but not datadoghq.com on their firewalls. This should fix that issue.

Review checklist

• PR has a meaningful title or PR has the no-changelog label attached
• Feature or bugfix has tests
• Git history is clean
• If PR impacts documentation, docs team has been notified or an issue has been opened on the documentation repo
• If PR adds a configuration option, it has been added to the configuration file.

Additional Notes

Anything else we should know when reviewing?

[codecov]

Codecov Report

Merging #3087 into master will increase coverage by 15.7%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #3087      +/-   ##
=========================================
+ Coverage   84.29%    100%   +15.7%     
=========================================
  Files         676       1     -675     
  Lines       37192      18   -37174     
  Branches     4431       2    -4429     
=========================================
- Hits        31350      18   -31332     
+ Misses       4570       0    -4570     
+ Partials     1272       0    -1272