Ansible 2.3 compatibility

[fullyint]

Broken out into separate commits, each with a commit message explaining. One commit prevents Ansible's new dense.py callback from causing Trellis ssh connection tests to always show root as failing to connect. The other commits just prevent Ansible's new warning message about jinja delimiters.

[WARNING]: when statements should not include jinja2 templating delimiters such as {{ }} or {% %}.

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The commit Avoid single var containing jinja delimiters in when parameter deserves some explanation but don't expect this explanation to stand on its own without examining the commit's diff.

Consider a when parameter with a single var, e.g., when: site_uses_letsencrypt. The var will be expanded and the var's jinja delimiters will trigger related warnings. This PR's commit avoids the warnings by removing jinja delimiters from the var value/definition:

# roles/letsencrypt/defaults.main.yml
- site_uses_letsencrypt: "{{ ssl_enabled and item.value.ssl.provider | default('manual') == 'letsencrypt' }}"
+ site_uses_letsencrypt: ssl_enabled and item.value.ssl.provider | default('manual') == 'letsencrypt'

That change prevents the warnings, but makes the site_uses_letsencrypt var not function in contexts that aren't part of the when task parameter. This accounts for why the commit in question removes site_uses_letsencrypt from the definition of missing_hosts

# roles/letsencrypt/defaults.main.yml
- missing_hosts: "{{ site_uses_letsencrypt | ternary(site_hosts, []) | difference(... bunch of stuff
+ missing_hosts: "{{ site_hosts | difference(... bunch of stuff

This change to missing_hosts means its two appearances in the form
when: missing_hosts | count must be adjusted. However, it turns out that the lack of jinja delimiters in site_uses_letsencrypt causes this adjustment to be ineffective:

- when: missing_hosts | count
+ when: missing_hosts | count and site_uses_letsencrypt

This accounts for why the commit in question uses the strategy below, leaving the site_uses_letsencrypt alone in a single item (vs. the compound item in the diff above).

- when: missing_hosts | count
+ when:
+   - site_uses_letsencrypt
+   - missing_hosts | count

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Tested on Ansible branch 2.3-stable as of 07ea6a6 and on Ansible 2.2.0.0. Tests included default configs and alternative configs such as Let's Encrypt enabled, nginx-includes, and root login disabled, on DigitalOcean and AWS EC2.

[fullyint]

This PR works with today's official release of Ansible 2.3.