Trust expected_version if provided

[vboginskey]

Fixes #2, perhaps controversially.

The reason it's not currently possible to provision, e.g., an RDS instance and modify it with a postgresql_* resource is that the postgresql provider attempts to parse the PostgreSQL version by connecting to the database during the planning phase.

This pull request allows the user to prevent this from happening by specifying the version directly via the existing expected_version parameter. In practice, you would do something like:

resource "aws_db_instance" "this" {
  [...]
}

provider "postgresql" {
  [...]
  expected_version = aws_db_instance.this.engine_version
}

The behavior of the provider if expected_version is not supplied remains the same.

[cyrilgdn]

@vboginskey Thanks for this PR, I'll take a look and test it this week!

I started to work on a fix #2 but it's a bit more complex than expected and I think meanwhile this fix could be a good workaround 👍 (and anyway it's a useful option)

Just need to check what was the goal of this expectedVersion too at the beginning 🤔

[cyrilgdn]

Thanks for your work, this little patch will help a lot!

[vboginskey]

Happy to help!