add associate elastic ip support

[suthio]

associate elastic ip support
after is not release elastic ip

before

aws.elastic_ip = true

after

aws.elastic_ip = '54.199.192.70'

[tonglil]

I'm going to try your patch out, this is a feature I've been looking for for a while now!

[LTangaF]

I'm interested in this as well. It is important to me to keep my Elastic IP upon termination and I'm glad I looked through the code before terminating an instance the first time I used this plugin.

I had Amazon associate a PTR record with my EIP and scrub it from mail blacklists, so I'm perfectly happy paying a couple dollars a month to maintain it when I'm not using it.

The code, as currently written, will release the EIP from my account, so I'm glad someone has submitted a PR with the option to allow it to be retained, though it looks like this PR code may give the flexibility to release it, it doesn't presently provide the ability to ask for that.