Support issuing/renewing SSH Host Certificates

[ckwalsh]

This pull request adds support for generating, distributing, and maintaining SSH Host certificates within smallstep/autocert. For ease of review, it is split into several diffs with various goals:

1. Update step dependencies to 0.15.3. I'm not sure exactly when SSH host support landed, but I figured having the latest version wasn't a bad idea.
2. Refactor the autocert controller to avoid assumptions it was working with TLS certificates. This logic was moved to structs that implement IdentityRequest, but the general structure/certificate issuing flow was not significantly changed.
3. Add support for SSH Host certs. This adds the SSHIdentityRequest to the controller, a new bootstrapper, and a new renewer.

These changes should be backwards compatible, and should not break existing deployments.

Future Work:

1. SSH renewer improvements. Unlike TLS certificates, step ssh doesn't have a renewal daemon mode. Instead, the renewer is a bash script that implements a while-true-sleep-work loop based on env vars. Assuming an ssh renewal daemon is added, it shouldn't be too hard to swap it in.
2. Documentation and config file updates. Especially need to fill in the SSHHost* fields in the autocert.yaml config.
3. SSH Client cert support. I implemented the code with the assumption it would be added, but didn't actually do so, as I don't need it. I'll leave it as an exercise to others.
4. Testing. I updated the unit tests to confirm no breakages, and with some tweaked config files, ran this on my homelab k8s install.

Notes:

1. This was my holiday project, and it was a lot of fun. I had heard of smallstep, and read yet another article about how certificates > passwords, and finally got around to diving into it a couple days before Christmas. Took a bit of time to grok the certificate distribution flow, but once I did, it's a pretty awesome project you guys built.
2. This is my first foray into Golang, the code was really easy to understand, modify, and build. Please point out if I broke any significant Go idioms.

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[codecov[bot]]

Codecov Report

Merging #24 (e31da80) into master (ef4f27c) will decrease coverage by 1.63%. The diff coverage is 11.73%.

@@            Coverage Diff            @@
##           master     #24      +/-   ##
=========================================
- Coverage    8.00%   6.36%   -1.64%     
=========================================
  Files           2       2              
  Lines         500     581      +81     
=========================================
- Hits           40      37       -3     
- Misses        459     544      +85     
+ Partials        1       0       -1     

Impacted Files	Coverage Δ
controller/main.go	6.95% <11.73%> (-2.04%)	:arrow_down:
controller/client.go	0.00% <0.00%> (ø)

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[ckwalsh]

Ping?

[maraino]

@ckwalsh Pong, sorry it went out of my radar. I will take a look asap.