step certificate create: Added subject as SAN if empty

[dopey]

Please enter the password to encrypt the private key:
Your certificate signing request has been saved in hello.csr.
Your private key has been saved in hello.key.
$ step ca sign hello.csr hello.crt
✔ Key ID: jO37dtDbku-Qnabs5VR0Yw6YFFv9weA18dp3htvdEjs ([email protected])
✔ Please enter the password to decrypt the provisioner key:
✔ CA: https://ca.smallstep.com:9000/1.0/sign
Unauthorized

Google Chrome version 58, released in April 2017, removed support for the X.509 certificate Subject Common Name (CN) as a source of naming information when validating certificates. As a result, certificates that do not carry all relevant domain names in the Subject Alternative Name (SAN) extension result in validation failures.

The step ca subcommands take this into account by adding the subject name as a SAN if no additional SANs are provided when generating a certificate. However, this behavior was not backfilled in the step certificate command group.

[codecov]

Codecov Report

Merging #87 into master will decrease coverage by 0.02%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master      #87      +/-   ##
==========================================
- Coverage   68.46%   68.44%   -0.03%     
==========================================
  Files          59       59              
  Lines        8008     8011       +3     
==========================================
  Hits         5483     5483              
- Misses       2161     2164       +3     
  Partials      364      364

Impacted Files	Coverage Δ
command/certificate/create.go	56.3% <0%> (-0.53%)	⬇️

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