Have a management command to set the admin password using envvar and/or file.

Right now we have the necessity to check if there's an admin password set and if it's not, set it using the content from a environment variable or from a file.

Currently we have this logic using a shell script:

if [[ -n "$PULP_DEFAULT_ADMIN_PASSWORD" ]]
then
  PASSWORD_SET=$(/usr/local/bin/pulpcore-manager shell -c "from django.contrib.auth import get_user_model; print(get_user_model().objects.filter(username=\"admin\").exists())")
  if [ "$PASSWORD_SET" = "False" ]
  then
    /usr/local/bin/pulpcore-manager reset-admin-password --password "${PULP_DEFAULT_ADMIN_PASSWORD}"
  fi
else
  ADMIN_PASSWORD_FILE=/etc/pulp/pulp-admin-password
  if [[ -f "$ADMIN_PASSWORD_FILE" ]]; then
     echo "pulp admin can be initialized."
     PULP_ADMIN_PASSWORD=$(cat $ADMIN_PASSWORD_FILE)
  fi

  if [ -n "${PULP_ADMIN_PASSWORD}" ]; then
      /usr/local/bin/pulpcore-manager reset-admin-password --password "${PULP_ADMIN_PASSWORD}"
  fi
fi

Moving the same logic used above to a management command would allow user from different contexts to use the feature.
Something like: pulpcore-manager set-init-password and then the magic happens.

Start by reviewing the existing reset-admin-password management command and the shell logic shown in the issue. Add a pulpcore-manager set-init-password entry point that checks for the admin user, reads PULP_DEFAULT_ADMIN_PASSWORD or /etc/pulp/pulp-admin-password, and initializes the password only when needed; verify both input paths and the already-initialized case.