allow spaces in db subnet name

[cbusbey]

fixes #3954

[radeksimko]

I think the comment above the test with links to other similar issues is worth more than any other comment.

Thanks for sending the patch! 👍

I will notify AWS. Apparently they have fixed AWS CLI docs last time when I was reporting dots & underscores: http://docs.aws.amazon.com/cli/latest/reference/rds/create-db-subnet-group.html but forgot completely about the API docs ^

[ghost]

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