provider/aws: Fix issue finding db subnets

[catsby]

AWS seems to lower case DB Subnet Group names, causing a failure in TF if your
name isn't all lower case.

This PR covers:

• lower casing both our saved ID and the name we get back from AWS, and then makes the comparison.
• range over all db subnets returned to look for the one in question
• changes the test to use upper case

[phinze]

Nice catch! Does this work okay given a fresh not-yet-created config and running terraform refresh? I can't remember whether the provider skips Read in that case. I was trying to think through it for the case where d.Id() == "", but maybe that's unnecessary.

[catsby]

Fixes #1376

[catsby]

Does this work okay given a fresh not-yet-created config and running terraform refresh

Running terraform refresh before the plan is applied returns nothing, so, yes?

[phinze]

Worked it out in chat - Read is not required to handle a case where d.Id() == "", that's handled at a higher level. So LGTM!

[ghost]

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