We pass the pillars via the template engine context, this avoid the
need to load `map.jinja` from the templates themselves and recude the
number of `pillar.get` calls.

* openssh/config.sls (sshd_config): pass `sshd_config` in the
  context.
  (ssh_config): pass `ssh_config` in the context.

* openssh/files/default/ssh_config: remove `map.jinja` import since
  it's now in the context.

* openssh/files/default/sshd_config: ditoo.

* openssh/known_hosts.sls: pass `known_hosts` in the context.

* openssh/files/default/ssh_known_hosts: use `known_hosts` from the
  context instead of calling `pillar.get` several times.

BREAKING CHANGE: Minimum Salt version support is now `2019.2` in line
with official upstream support; also use of the `traverse` Jinja filter.

* Use consistent Jinja whitespace control `{%- ... -}`
* Improve debug output (comments & whitespace control)
* Use exact state names with TOFS `files_switch`
* Add `ssh_known_hosts_src` to `defaults` (for consistency)
* Restrict `pillar.example` changes to TOFS only
* Use `fire_banner` in `pillar.example` to indicate available template

This state manages /etc/ssh/ssh_known_hosts and fills it with
public SSH host keys of other minions.