postgresql_server: add conditional for small postgresql instances

The recent changes to postgresql_server increased the required memory past
what one of my VMs had. I've added a conditional in postgresql.conf to put
some memory settings back where they used to be (controlled by
small_postgres_instance, default is false) and created a default to not use
the small_postgres_instance settings unless specified

roles/postgresql_server/default/main.yml

@@ -0,0 +1,2 @@
+---
+small_postgres_instance: false

roles/postgresql_server/templates/postgresql.conf

@@ -121,8 +121,13 @@ temp_buffers = 32MB                     # min 800kB
 
 # 8 MB is probably on the high side.  We can probably do with 4MB.  But we
 # were seeing a problem and we have the RAM so we're going to try this.   
+{% if small_postgres_instance %}
+work_mem = 4MB
+maintenance_work_mem = 1GB              # min 1MB
+{% else %}
 work_mem = 32MB
 maintenance_work_mem = 2GB              # min 1MB
+{% endif %}
 #max_stack_depth = 2MB                  # min 100kB                       
 
 # - Free Space Map -