Make environment variables optional for tagged hosts

[nbgoodall]

Hey,

When deploying with tagged servers, e.g.:

servers:
  job:
    hosts:
      - 10.100.0.4: job
    proxy: false
    cmd: bin/jobs

...the deploy would fail at the end with undefined method 'env' for nil if env.tags.job wasn't set.

This PR fixes that by compacting the env_tags in lib/kamal/configuration/role.rb, along with a little test.