Allow to configure oslo.cache settings

This patch ensures we enable cache when _data.servers or _data.members is defined.
Also unhardcode backend setting by using _data.backend pillar

Change-Id: I962f961cc8d4425ecfaddf85a20f7ce15adf9c64
diff --git a/oslo_templates/files/queens/oslo/_cache.conf b/oslo_templates/files/queens/oslo/_cache.conf
index 6ae46ad..8a41354 100644
--- a/oslo_templates/files/queens/oslo/_cache.conf
+++ b/oslo_templates/files/queens/oslo/_cache.conf
@@ -35,6 +35,7 @@
 # dogpile.cache.memory_pickle - <No description provided>
 # dogpile.cache.null - <No description provided>
 #backend = dogpile.cache.null
+backend = {{ _data.get('backend', 'oslo_cache.memcache_pool') }}
 
 # Arguments supplied to the backend module. Specify this option once
 # per argument to be passed to the dogpile.cache backend. Example
@@ -48,6 +49,15 @@
 
 # Global toggle for caching. (boolean value)
 #enabled = false
+{%- if _data.enabled is defined %}
+  {%- if _data.enabled %}
+enabled = True
+  {%- endif %}
+{%- else %}
+  {%- if _data.servers is defined or _data.members is defined %}
+enabled = True
+  {%- endif %}
+{%- endif %}
 
 # Extra debugging from the cache backend (cache keys,
 # get/set/delete/etc calls). This is only really useful if you need to