Stups-senza

- 310: add ELBv2 support for `senza health`
- 310: fix Security Groups for Application Load Balancer

- 334: fix SSL certificate handling (was always `null`)

- 310: allow overwriting ELBv2 CF properties
- 331: allow overwriting ASG properties (e.g. to enable CloudWatch ASG metrics)
- 330: truncate CF `Description` to 1024 chars

- 310: first draft of ELBv2 support (application load balancer)

- 321: switch traffic via Cloud Formation update (instead of direct Route53 manipulation)
- 323: `--piu`: don't connect to instance directly if more than one instance listed
- 320: allow adding custom tags to AutoScalingGroup
- 308: fix potential infinite loop in `senza wait`

- 88: implement first draft of `senza health`