aws_eip: dissociate EIP on update

[catsby]

Fixes #42 by dissociating the EIP before trying to update it. Only do this on true update and not part of the create, as there is not actual association yet. Includes regression test TestAccAWSEIP_classic_disassociate, which fails on master

Originally reported at hashicorp/terraform#3429

=== RUN   TestAccAWSEIP_basic
--- PASS: TestAccAWSEIP_basic (11.72s)
=== RUN   TestAccAWSEIP_instance
--- PASS: TestAccAWSEIP_instance (276.85s)
=== RUN   TestAccAWSEIP_network_interface
--- PASS: TestAccAWSEIP_network_interface (35.14s)
=== RUN   TestAccAWSEIP_twoEIPsOneNetworkInterface
--- PASS: TestAccAWSEIP_twoEIPsOneNetworkInterface (36.33s)
=== RUN   TestAccAWSEIP_associated_user_private_ip
--- PASS: TestAccAWSEIP_associated_user_private_ip (153.29s)
=== RUN   TestAccAWSEIP_classic_disassociate
--- PASS: TestAccAWSEIP_classic_disassociate (215.83s)

[radeksimko]

Except one mentioned nitpick this LGTM.

[radeksimko]

I think the conditional here is superfluous as we're resetting err to nil below. We can just return err

[ghost]

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