Lambda deployment improvement - ensure requirement dir in PYTHONPATH

[yubozhao]

Ensure /tmp/requirements directory is in PYTHONPATH.

[codecov-io]

Codecov Report

Merging #508 into master will increase coverage by 0.13%.
The diff coverage is 22.22%.

@@            Coverage Diff             @@
##           master     #508      +/-   ##
==========================================
+ Coverage   56.29%   56.42%   +0.13%     
==========================================
  Files          89       89              
  Lines        6402     6408       +6     
==========================================
+ Hits         3604     3616      +12     
+ Misses       2798     2792       -6

Impacted Files	Coverage Δ
.../deployment/aws_lambda/download_extra_resources.py	0% <0%> (ø)	⬆️
bentoml/deployment/aws_lambda/lambda_app.py	91.17% <100%> (+0.55%)	⬆️
bentoml/handlers/dataframe_handler.py	83.33% <0%> (-0.71%)	⬇️
bentoml/yatai/yatai_service_impl.py	35.18% <0%> (ø)	⬆️
bentoml/configuration/__init__.py	88.73% <0%> (+1.4%)	⬆️
bentoml/_version.py	44.4% <0%> (+1.8%)	⬆️
bentoml/bundler/config.py	94.5% <0%> (+3.29%)	⬆️

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