Improve workflow for upgrade

[z-wenlin]

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This PR improves the workflow for using upgrade tools.

1. running the command "el8_migrate_locale" after sourcing "greenplum_path.sh", there is no need to specify "python" nor a ".py" file extension to execute the utility.

2. execute "el8_migrate_locale identify" and run a check of an existing Greenplum cluster for impacted index and tables, and it will output STDOUT information.

3. execute "el8_migrate_locale prepare --output [FILE]" prepare for an upgrade, and it will output to the given [FILE] the commands for migrating BOTH the impacted index and partitioned tables.

4. execute "el8_migrate_locale validate" will validate after migration that data correctness of partitioned tables.

Here are some reminders before you submit the pull request

• [ ] Add tests for the change
• [ ] Document changes
• [ ] Communicate in the mailing list if needed
• [ ] Pass make installcheck
• [ ] Review a PR in return to support the community