sagemaker-inference-toolkit fix: write the mms config file to a subdir of base

fix #72

Issue #, if available: 72

Description of changes:

write the mms config file to a subdir os base_dir instead of /etc

Testing done:

unit tests pass on local

Merge Checklist

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General

[X] I have read the CONTRIBUTING doc
[X] I used the commit message format described in CONTRIBUTING
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[x] I have updated any necessary documentation, including READMEs

Tests

[ ] I have added tests that prove my fix is effective or that my feature works (if appropriate)
[X] I have checked that my tests are not configured for a specific region or account (if appropriate)

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