Enable tensorflow for content detection in a production build

[cyberj0g]

Changes:

• install_ffmpeg.sh now installs Tensorflow library and enables it for Ffmpeg without experimental BUILD_TAGS (no side effects when running in non-Nvidia environment)
• Dockerfile copies Tensorflow distribution files to final container, because libtensorflow.so is loaded at runtime by Ffmpeg DNN filter and is not linked statically inside livepeer binary, like the rest of dependencies

Kudos to @hjpotter92 for helping to figure out Docker stuff.

[codecov]

Codecov Report

Merging #2695 (b88490a) into master (3d4bce7) will increase coverage by 0.00980%.
The diff coverage is 0.00000%.

@@                 Coverage Diff                 @@
##              master       #2695         +/-   ##
===================================================
+ Coverage   56.34804%   56.35784%   +0.00980%     
===================================================
  Files             88          88                 
  Lines          19124       19126          +2     
===================================================
+ Hits           10776       10779          +3     
+ Misses          7760        7759          -1     
  Partials         588         588                 

Impacted Files	Coverage Δ
core/orchestrator.go	77.65517% <0.00000%> (-0.21482%)	⬇️
discovery/db_discovery.go	70.75812% <0.00000%> (+1.08303%)	⬆️

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Impacted Files	Coverage Δ
core/orchestrator.go	77.65517% <0.00000%> (-0.21482%)	⬇️
discovery/db_discovery.go	70.75812% <0.00000%> (+1.08303%)	⬆️