Fix AWS security/session token not being set

[alej0varas]

Changed a little the logic for setting backends.s3boto.S3BotoStorage and backends.s3boto3.S3Boto3Storage attributes access_key, secret_key and security_token.
Currently if access_key or secret_key are set security_token is not set.
Code is tested on Lambda.

Fix #282
Related to #283 and #508

[codecov-io]

Codecov Report

Merging #514 into master will decrease coverage by 0.02%.
The diff coverage is 95%.

@@            Coverage Diff             @@
##           master     #514      +/-   ##
==========================================
- Coverage   76.48%   76.46%   -0.03%     
==========================================
  Files          11       11              
  Lines        1595     1589       -6     
==========================================
- Hits         1220     1215       -5     
+ Misses        375      374       -1

Impacted Files	Coverage Δ
storages/backends/s3boto.py	88.13% <100%> (+0.09%)	⬆️
storages/backends/s3boto3.py	87.19% <100%> (+0.06%)	⬆️
storages/utils.py	94.73% <83.33%> (-2.14%)	⬇️

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[jschneier]

Thanks! Merged in b13efd9.