Allow underscores in aws_db_subnet_group name

[CpuID]

Docs don't claim they are allowed but they are.

They don't explicitly say they are not allowed... but yea.

http://docs.aws.amazon.com/AmazonRDS/latest/CommandLineReference/CLIReference-cmd-ModifyDBSubnetGroup.html

Constraints: Must contain from 1 to 255 alphanumeric characters or hyphens. First character must be a letter. Cannot end with a hyphen or contain two consecutive hyphens.

For #2603

[radeksimko]

Interesting to see the mismatch between AWS docs and reality... :-) I will notice them.

It would be nice if we can test this somehow, i.e. verify that AWS actually permits that and more importantly keep verifying per each release that this behaviour doesn't change (as it's undocumented).
I know RDS instance may take ~30mins+ to create/update/delete, that would be too expensive for a simple name validation test, but it's worth checking how long does it take to create & destroy subnet. If it's quick, I'd rather have that test in place.

Would you mind having a look and eventually adding that test (or confirming it takes ages to create/destroy)? If you don't have time, that's ok, I can do it.

[CpuID]

I'll see if I have time to get through this today, see how I go. Will ping you if I get stuck.

[radeksimko]

I will add the test after merging.

[ghost]

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