Add support for alternative AWS partitions - GovCloud, etc.

The PMM autodiscovery of RDS instances iterates only through default AWS partition, and there is no way to switch to special regions, like the GovCloud one.

We need support for alternative regions.
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RDS Discovery UI will work based on the value of the setting "aws_partitions"
The default value is "aws".
Available values:

• "aws" - Public AWS partition
• "aws-cn" - AWS China
• "aws-us-gov" - AWS GovCloud

You can use several of them, JSON Array. ex: ["aws", "aws-cn"]

Based on the lists of partitions (https://docs.aws.amazon.com/general/latest/gr/aws-arns-and-namespaces.html) PWW will perform a lookup for instances.