Security/Watcher: Increase index priority for indices (elastic/elasticsearch#3709)

The `.triggered-watches`, `.watches` and `.security` indices should load
as early as possible, and not wait for other indices (especially not
for time-based indices, that are old).

This commit adds an index.priority to the template for those indices.
The values 1000, 900 and 800 were chosen rather arbitrary, mainly we
did not want to go with 10, because it was used in the sample documentation.

Security should always be loaded first, because we might need this index for
other operations.

Any administrator can still change all the values in the indices, but this
cares for better defaults.

Original commit: elastic/x-pack-elasticsearch@6ed0fb7975

elasticsearch/src/main/resources/security-index-template.json

@@ -5,6 +5,7 @@
     "number_of_shards" : 1,
     "number_of_replicas" : 0,
     "auto_expand_replicas" : "0-all",
+    "index.priority": 1000,
     "analysis" : {
       "filter" : {
         "email" : {

elasticsearch/src/main/resources/triggered_watches.json

@@ -4,7 +4,8 @@
   "settings": {
     "index.number_of_shards": 1,
     "index.mapper.dynamic" : false,
-    "index.refresh_interval" : "-1"
+    "index.refresh_interval" : "-1",
+    "index.priority": 900
   },
   "mappings": {
     "triggered_watch": {

elasticsearch/src/main/resources/watches.json

@@ -3,7 +3,8 @@
   "order": 2147483647,
   "settings": {
     "index.number_of_shards": 1,
-    "index.mapper.dynamic" : false
+    "index.mapper.dynamic" : false,
+    "index.priority": 800
   },
   "mappings": {
     "watch": {