feat(templates): don't get openssh pillars in templates

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.

openssh/known_hosts.sls

@@ -14,6 +14,8 @@ manage ssh_known_hosts file:
                                'manage ssh_known_hosts file'
               ) }}
     - template: jinja
+    - context:
+        known_hosts: {{ openssh | traverse("known_hosts", {}) }}
     - user: root
     - group: {{ openssh.ssh_config_group }}
     - mode: 644