Add retries to gpssh and allow to failfast when hosts not reachable

[ostinru]

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Hi, greenplum team!

Here a patch that:

• adds retries to connection phase of gpssh
• allows to fail fast when some of hosts are not reachable

I am using this to make some scripting around greenplum a bit more reliable.

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