enhancement: Alt credentials location support

[fstab50]

Originally reported by: Blake Huber (Bitbucket: blakeca00, GitHub: Unknown)

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Upon startup, check the $AWS_SHARED_CREDENTIALS_FILE variable for an alternate location of the
awscli credentials dir. Set cred_path = value if found; else use default location (~/.aws).
From a cli, see:

    $ aws help config-vars

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• Bitbucket: https://bitbucket.org/blakeca00/stsaval/issue/8

[fstab50]

Original comment by Blake Huber (Bitbucket: blakeca00, GitHub: Unknown):

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staval currently (v0.1.8) has built-in support for $AWS_SHARED_CREDENTIALS_FILE, if present in global vars environment