Show a deprecation warning for --life-span=0

[asvetlov]

An explicit positive value should be passed to avoid resource leakage.

The value could be arbitrarily big, feel free to pass 365d if it is required really.

[codecov]

Codecov Report

Merging #1749 into master will decrease coverage by 0.16%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1749      +/-   ##
==========================================
- Coverage   87.40%   87.24%   -0.17%     
==========================================
  Files          62       62              
  Lines        9061     9063       +2     
  Branches     1463     1463              
==========================================
- Hits         7920     7907      -13     
- Misses        893      905      +12     
- Partials      248      251       +3     

Flag	Coverage Δ
#e2e	65.56% <50.00%> (-0.15%)	⬇️
#unit	77.49% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
neuromation/cli/utils.py	89.11% <100.00%> (+0.06%)	⬆️
neuromation/cli/ael.py	46.19% <0.00%> (-3.81%)	⬇️

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