validate-env command?

[stig]

We have validate template, but for CI it would be very useful to have a validate-env command. Most of our projects contains a variant of this snippet in their .circleci/config.yaml:

      - run:
          name: Validate templates in dev using Sceptre
          command: |
            for file in $(find config -path '*/dev/*' -type f -name '*.yaml' -not -name config.yaml) ; do
                stack=$(basename $file ".yaml")
                environment=$(dirname $file | sed 's,.*config/,,')
                echo ">>>> sceptre validate-template $environment $stack"
                venv/bin/sceptre validate-template $environment $stack
            done

It would be very nice, IMO, if we could instead do:

    - run: sceptre validate-env dev

[cbosss]

Similar to #69

[stig]

@cboss24 Indeed! Mea culpa, I should have searched more carefully before opening this.