Allow 1 as true value in environment variables

[marcotc]

We currently only support the string true as a truthy boolean configuration value (case insensitive).

Our standard for APM Tracing configuration is actually to allow either true or 1.

This PR changes the logic to accept 1 as a valid true value for environment variable configuration.

There are no other functional changes to configuration parsing.

[codecov-commenter]

Codecov Report

Merging #2710 (84b3af3) into master (c8f02ef) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2710      +/-   ##
==========================================
- Coverage   98.06%   98.06%   -0.01%     
==========================================
  Files        1206     1206              
  Lines       66207    66212       +5     
  Branches     2958     2959       +1     
==========================================
+ Hits        64925    64928       +3     
- Misses       1282     1284       +2     

Impacted Files	Coverage Δ
lib/datadog/core/environment/variable_helpers.rb	100.00% <100.00%> (ø)
.../datadog/core/environment/variable_helpers_spec.rb	100.00% <100.00%> (ø)

... and 3 files with indirect coverage changes

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