implementing retro-compatible behavior

Using the old salt.engines pillar and merging it with the new
salt.[master|minion].engines pillar.
This way, it doesn't break previous behavior and permits to define
common engines on master and minion.
In the merge, the salt.[master|minion].engines pillar takes precedence
if conflict as it's the more specific pillar.

salt/files/master.d/engine.conf

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