support custom env vars for etcd

roles/etcd/defaults/main.yml

@@ -31,6 +31,12 @@ etcd_election_timeout: "5000"
 
 etcd_metrics: "basic"
 
+## A dictionary of extra environment variables to add to etcd.env, formatted like:
+##  etcd_extra_vars:
+##    ETCD_VAR1: "value1"
+##    ETCD_VAR2: "value2"
+etcd_extra_vars: {}
+
 # Limits
 # Limit memory only if <4GB memory on host. 0=unlimited
 etcd_memory_limit: "{% if ansible_memtotal_mb < 4096 %}512M{% else %}0{% endif %}"