Fix bug in feast alpha enable CLI command

[felixwang9817]

Signed-off-by: Felix Wang wangfelix98@gmail.com

What this PR does / why we need it: Previously, feast alpha enable could modify the feature_store.yaml file in a breaking way; this PR prevents that from happening. Note that non-breaking, but potentially unexpected modifications, are still possible. For example, calling feast alpha enable python_feature_server on the following feature_store.yaml

registry: data/registry.db
project: smooth_fish
provider: local
online_store:
  type: redis
flags:
  alpha_features: true
  python_feature_server: true

will still change it to

registry: data/registry.db
project: smooth_fish
provider: local
online_store:
  type: redis
offline_store:
  type: file
flags:
  alpha_features: true
  python_feature_server: true

Which issue(s) this PR fixes:

Fixes #1938

Does this PR introduce a user-facing change?:

The feast alpha enable CLI command is now fixed

[felixwang9817]

/kind bug

[codecov-commenter]

Codecov Report

Merging #1940 (65abaa9) into master (e8a1519) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1940   +/-   ##
=======================================
  Coverage   82.26%   82.26%           
=======================================
  Files          96       96           
  Lines        7669     7669           
=======================================
  Hits         6309     6309           
  Misses       1360     1360           

Flag	Coverage Δ
integrationtests	74.15% <ø> (ø)
unittests	59.39% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
sdk/python/feast/repo_config.py	78.97% <ø> (ø)

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[achals]

/lgtm

[feast-ci-bot]

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