Creating autoscaling groups should be idempotent

[rossf7]

After fixing #323 the operator now errors if the workers autoscaling group already exists.

"caller":"github.com/giantswarm/aws-operator/service/create/service.go:915","error":"[{/Users/ross/.gvm/pkgsets/go1.8/giantswarm/src/github.com/giantswarm/aws-operator/resources/aws/auto_scaling_group.go:70: } {AlreadyExists: AutoScalingGroup by this name already exists - A group with the name ross-cluster-worker already exists\n\tstatus code: 400, request id: 15756124-7053-11e7-9137-dbbefd641799}]","time":"17-07-24 09:33:10.779"}

Creating ASGs should also be idempotent. This should be the last of these errors.