feat: make deployment_config.yaml file default if -f flag not given

[jjmachan]

Description

Motivation and Context

How Has This Been Tested?

Checklist:

• My code follows the bentoctl code style, both make format and
  make lint script have passed
• My change reduces project test coverage and requires unit tests to be added
• I have added unit tests covering my code change
• My change requires a change to the documentation
• I have updated the documentation accordingly

[codecov]

Codecov Report

Merging #121 (8bc7216) into main (793be1c) will increase coverage by 0.04%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #121      +/-   ##
==========================================
+ Coverage   39.67%   39.71%   +0.04%     
==========================================
  Files          15       15              
  Lines         852      851       -1     
==========================================
  Hits          338      338              
+ Misses        514      513       -1     

Flag	Coverage Δ
unit-tests	39.71% <0.00%> (+0.04%)	⬆️

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Impacted Files	Coverage Δ
bentoctl/cli/__init__.py	0.00% <0.00%> (ø)

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