make the configuration of engines specific to master or minion

the engines are now configured using the following pillars:

* salt.master.engines
* salt.minion.engines

instead of a global salt.engines pillar.

Note: the pillar.example provided seems to assume this behaviour.
(the pillar is salt.master.engines.slack and not salt.engines.slack)

salt/files/master.d/engine.conf

 #
 # This file is managed by Salt! Do not edit by hand!
 #
-{%- set engines = salt['pillar.get']('salt:engines') -%}
+{%- set engines = salt['pillar.get']('salt:master:engines') -%}
 {%- if engines %}
 engines:
   {{ engines | yaml(False) | indent(2) }}

salt/files/minion.d/engine.conf

 #
 # This file is managed by Salt! Do not edit by hand!
 #
-{%- set engines = salt['pillar.get']('salt:engines') -%}
+{%- set engines = salt['pillar.get']('salt:minion:engines') -%}
 {%- if engines %}
 engines:
   {{ engines | yaml(False) | indent(2) }}