Update download_live_ocean re: sentinel file chg

[douglatornell]

Parker's automation change on 9-Nov-2021 changed the content of the sentinel
file that indicates that the netCDF file is ready to download.
New file contains just the date/time when the netCDF file is ready.

Also, improve production config unit tests.

[codecov]

Codecov Report

Merging #84 (cdf9e8a) into main (7af9425) will increase coverage by 0.02%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##             main      #84      +/-   ##
==========================================
+ Coverage   72.89%   72.91%   +0.02%     
==========================================
  Files         122      122              
  Lines       15984    15982       -2     
  Branches      956      953       -3     
==========================================
+ Hits        11651    11653       +2     
+ Misses       4283     4279       -4     
  Partials       50       50              

Flag	Coverage Δ
unittests	72.91% <66.66%> (+0.02%)	⬆️

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Impacted Files	Coverage Δ
nowcast/workers/download_live_ocean.py	78.94% <0.00%> (+3.94%)	⬆️
tests/workers/test_download_live_ocean.py	100.00% <100.00%> (ø)

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