How to load API_KEY from a config file instead of environment variable

[nghialv]

**Output of the info page **

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Steps to reproduce the issue:
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Describe the results you received:

At our project, we avoid using the environment variable for configuring all sensitive data.
Can we load API_KEY from a config file currently?

Describe the results you expected:

Additional information you deem important (e.g. issue happens only occasionally):

[hkaj]

Hey @nghialv I replied in DataDog/datadog-agent#3235 - let's continue the conversation there.