Add ability to specify basic auth creds via environment variable

Useful for testing and running against a local GraceDB instance.
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Status Name Job ID Coverage
  Build
passed build:debian:jessie #534408

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passed build:debian:stretch #534409

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passed build:el7 #534407

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passed build:tarball #534406

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  Test
passed docs #534415

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passed syntax #534416

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passed test:python2.7 #534410

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passed test:python3.4 #534411

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passed test:python3.5 #534412

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passed test:python3.6 #534413

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passed test:python3.7 #534414

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