fix: set the defaults for shared_fs mount in genai correctly

[tayritenour]

Ticket

Description

We were not setting the default for the shared_fs mount dir correctly in det deploy aws --deployment-type genai

Test Plan

Deploy a cluster with genai:

det deploy aws up \
    --deployment-type genai \
    --genai-version 0.1.12 \
    --cluster-id YOUR_CLUSTER_HERE \
    --keypair YOUR_KEYPAIR \
    --det-v 0.32.1 \
    --aux-agent-instance-type m5.large \
    --compute-agent-instance-type p3.16xlarge

The master node should have no errors in the efs build in /var/log/cloud-init-output.log nor when mounting to the docker container

Checklist

• Changes have been manually QA'd
• User-facing API changes need the "User-facing API Change" label.
• Release notes should be added as a separate file under docs/release-notes/.
  See Release Note for details.
• Licenses should be included for new code which was copied and/or modified from any external code.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 48.61%. Comparing base (9524dd4) to head (cded388).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9433      +/-   ##
==========================================
- Coverage   51.24%   48.61%   -2.64%     
==========================================
  Files         746     1233     +487     
  Lines      110663   158973   +48310     
  Branches     2778     2778              
==========================================
+ Hits        56712    77278   +20566     
- Misses      53777    81521   +27744     
  Partials      174      174              

Flag	Coverage Δ
backend	42.16% <ø> (+7.88%)	⬆️
harness	64.02% <ø> (ø)
web	44.39% <ø> (ø)

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Files	Coverage Δ
harness/determined/deploy/aws/cli.py	16.86% <ø> (ø)

... and 488 files with indirect coverage changes

[hamidzr]

lgtm. just noting that adding the default to the cli arg could have side-effects in terms of not being able to tell if the user has provided it or not which matters at least for some of the other arguments