add ELB support to Dokku stack

[tobiasmcnulty]

Fixes: #23

@vkurup This is the simplest possible change I could think of to add support for an ELB, though I'm on the fence about whether this or the "fully featured" option (with multiple Dokku instances and an auto scaling group) is a better default. We could still do the latter but keep the default instance count at 1. I feel like the EC2 instances option and the Dokku option are similar conceptually and should probably use much of the same code (whether that includes the auto scaling group or not).

One complicating factor is that supporting N instances without an auto scaling group is somewhat of a mess: https://stackoverflow.com/questions/5251934/how-to-create-variable-number-of-ec2-instance-resources-in-cloudformation-templa

Updated 10/13/17: I've switched this to use a load balancer and auto scaling group all the time. It's nice to have a DNS name (for the ELB) to point things to, in addition to the SSL cert of course.

[vkurup]

Hmmm, yeah, it does sound like having an ELB with an ASG might be a better default. I still haven't wrapped my head around Troposphere/CloudFormation, so I can't provide much constructive feedback on this PR (yet!). But I can take a deeper dive when you have more for me to review.

[vkurup]

I've tried this out and I can kinda get it to work, but wondering if I've misconfigured things. I created a stack using the domain name vktest29.kurup.com. I approved the cert via email and everything got created properly. I pointed the domain at the load balancer using a CNAME. I enabled the 'through-the-web' dokku setup, so I loaded vktest29.kurup.com (via HTTPS, woohoo!) and was able to successfully submit the setup form (using the numeric IP of the EC2 instance, which is what the form defaulted to... wonder if this was a misconfiguration on my part)

I ran into the following issues:

1. I can't run dokku commands against the vktest29.kurup.com domain. I think this is because the LB only accepts port 80 and 443 connections, so it times out when trying to connect via SSH.
2. I can successfully SSH into the EC2 instance behind the load balancer, but dokku commands don't run. I think it's because of this AWS-specific misconfigured authorized_keys file. When I fix the file, then I can run dokku commands normally.

[tobiasmcnulty]

Closing this to keep Dokku support simple for now...we might even want to sunset it altogether.