provider/aws: Switch to session.Copy to build up configs
provider/aws: Switch to session.Copy to build up configs

Found this function which makes the config setup much nicer - looks like
it's meant to be used for the very thing we're doing, which is starting
w/ a base config and customizing it slightly.

https://docs.aws.amazon.com/sdk-for-go/api/aws/session/Session.html#Copy-instance_method

This will make the User-Agent setting code to follow much simpler.

nbaztec/terraform-provider-aws,ClementCunin/terraform-provider-aws,nbaztec/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,ClementCunin/terraform-provider-aws,kjmkznr/terraform-provider-aws,Ninir/terraform-provider-aws,terraform-providers/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,ClementCunin/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,ashinohara/terraform-provider-aws,nbaztec/terraform-provider-aws,Ninir/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws,ashinohara/terraform-provider-aws,ClementCunin/terraform-provider-aws,Ninir/terraform-provider-aws,terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws,ashinohara/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,kjmkznr/terraform-provider-aws,nbaztec/terraform-provider-aws,ashinohara/terraform-provider-aws,Ninir/terraform-provider-aws